[Core] Improve detokenization performance for prefill (#3469)

Co-authored-by: MeloYang <meloyang05@gmail.com>

tests/tokenization/test_detokenize.py

@@ -1,13 +1,17 @@
 import pytest
 
 from transformers import AutoTokenizer
+from typing import List, Dict
 
+from vllm.sequence import Sequence, Logprob, SamplingParams, SequenceGroup
+from vllm.transformers_utils.tokenizer_group import get_tokenizer_group
 from vllm.transformers_utils.tokenizer import detokenize_incrementally
+from vllm.transformers_utils.detokenizer import Detokenizer
 
 TRUTH = [
-    "Hello here, this is a simple test",  # noqa: E501
+    "Hello here, this is a simple test",
-    "vLLM is a high-throughput and memory-efficient inference and serving engine for LLMs. It is designed to be used in production environments, where inference and serving",  # noqa: E501
+    "vLLM is a high-throughput and memory-efficient inference and serving engine for LLMs. It is designed to be used in production environments, where inference and serving",  # noqa
-    "我很感谢你的热情"  # noqa: E501
+    "我很感谢你的热情"
 ]
 TOKENIZERS = [
     "facebook/opt-125m",
@@ -24,12 +28,12 @@ TOKENIZERS = [
 
 
 def _run_incremental_decode(tokenizer, all_input_ids,
-                            skip_special_tokens: bool):
+                            skip_special_tokens: bool, starting_index: int):
     decoded_text = ""
     offset = 0
     token_offset = 0
     prev_tokens = None
-    for i in range(len(all_input_ids)):
+    for i in range(starting_index, len(all_input_ids)):
         new_tokens, text, offset, token_offset = detokenize_incrementally(
             tokenizer,
             all_input_ids[:i + 1],
@@ -46,17 +50,152 @@ def _run_incremental_decode(tokenizer, all_input_ids,
 
 
 @pytest.mark.parametrize("truth", TRUTH)
+@pytest.mark.parametrize("with_prompt", [True, False])
 @pytest.mark.parametrize("tokenizer_id", TOKENIZERS)
 @pytest.mark.parametrize("skip_special_tokens", (True, False))
-def test_decode_streaming(tokenizer_id, truth, skip_special_tokens):
+def test_decode_streaming(tokenizer_id, truth, with_prompt,
+                          skip_special_tokens):
     tokenizer = AutoTokenizer.from_pretrained(tokenizer_id)
-    all_input_ids = tokenizer(truth, add_special_tokens=False)["input_ids"]
+    if with_prompt:
+        truth_tokens = tokenizer(truth, add_special_tokens=False)["input_ids"]
+        prompt_input_ids = truth_tokens[:len(truth) // 2]
+        generated_input_ids = truth_tokens[len(truth) // 2:]
+        all_input_ids = prompt_input_ids + generated_input_ids
+        starting_index = len(prompt_input_ids)
+        prompt = tokenizer.decode(prompt_input_ids,
+                                  skip_special_tokens=skip_special_tokens)
+        generated = truth[len(prompt):]
+    else:
+        generated = truth
+        starting_index = 0
+        all_input_ids = tokenizer(truth, add_special_tokens=False)["input_ids"]
     if skip_special_tokens:
-        all_input_ids = ([tokenizer.bos_token_id]
+        if tokenizer.bos_token_id is not None:
-                         if tokenizer.bos_token_id is not None else
+            all_input_ids = [tokenizer.bos_token_id] + all_input_ids
-                         []) + all_input_ids + [tokenizer.eos_token_id]
+            starting_index += 1
+        all_input_ids = all_input_ids + [tokenizer.eos_token_id]
 
     decoded_text = _run_incremental_decode(
-        tokenizer, all_input_ids, skip_special_tokens=skip_special_tokens)
+        tokenizer,
+        all_input_ids,
+        skip_special_tokens=skip_special_tokens,
+        starting_index=starting_index)
 
-    assert decoded_text == truth
+    assert decoded_text == generated
+
+
+@pytest.fixture
+def detokenizer(tokenizer_name: str) -> Detokenizer:
+    init_kwargs = dict(
+        tokenizer_id=tokenizer_name,
+        enable_lora=False,
+        max_num_seqs=100,
+        max_input_length=None,
+        tokenizer_mode="auto",
+        trust_remote_code=False,
+        revision=None,
+    )
+
+    tokenizer_group = get_tokenizer_group(
+        None,
+        **init_kwargs,
+    )
+
+    return Detokenizer(tokenizer_group)
+
+
+@pytest.fixture(name="complete_sequence_token_ids")
+def create_complete_sequence_token_ids(complete_sequence: str,
+                                       tokenizer_name: str) -> List[int]:
+    tokenizer = AutoTokenizer.from_pretrained(tokenizer_name)
+    complete_sequence_token_ids = tokenizer(complete_sequence)["input_ids"]
+    return complete_sequence_token_ids
+
+
+def create_sequence(prompt_token_ids=None):
+    prompt_token_ids = prompt_token_ids or [1]
+    return Sequence(
+        seq_id=0,
+        prompt="<s>",
+        prompt_token_ids=prompt_token_ids,
+        block_size=16,
+    )
+
+
+def create_dummy_logprobs(
+        complete_sequence_token_ids: List[int]) -> List[Dict[int, Logprob]]:
+    return [{
+        token_id: Logprob(logprob=0.0),
+        token_id + 1: Logprob(logprob=0.1)
+    } for token_id in complete_sequence_token_ids]
+
+
+@pytest.mark.parametrize("complete_sequence", TRUTH)
+@pytest.mark.parametrize("tokenizer_name", TOKENIZERS)
+@pytest.mark.parametrize("skip_special_tokens", [True, False])
+def test_decode_sequence_logprobs(complete_sequence: str,
+                                  complete_sequence_token_ids: List[int],
+                                  detokenizer: Detokenizer,
+                                  skip_special_tokens: bool):
+    """Verify Detokenizer decodes logprobs correctly."""
+    sampling_params = SamplingParams(skip_special_tokens=skip_special_tokens,
+                                     logprobs=2)
+
+    # Run sequentially.
+    seq = create_sequence()
+    dummy_logprobs = create_dummy_logprobs(complete_sequence_token_ids)
+    sequential_logprobs_text_chosen_token = []
+    sequential_logprobs_text_other_token = []
+    for new_token, logprobs in zip(complete_sequence_token_ids,
+                                   dummy_logprobs):
+        seq.append_token_id(new_token, logprobs)
+        detokenizer.decode_sequence_inplace(seq, sampling_params)
+        sequential_logprobs_text_chosen_token.append(
+            seq.output_logprobs[-1][new_token].decoded_token)
+        sequential_logprobs_text_other_token.append(
+            seq.output_logprobs[-1][new_token + 1].decoded_token)
+    sequential_result = seq.output_text
+
+    assert sequential_result == "".join(sequential_logprobs_text_chosen_token)
+    assert sequential_result != "".join(sequential_logprobs_text_other_token)
+
+    if skip_special_tokens:
+        # Text for logprobs for the chosen token should be the same as the
+        # generated text. Note that this will only be true if we skip
+        # special tokens.
+        assert sequential_result == complete_sequence
+
+
+@pytest.mark.parametrize("complete_sequence", TRUTH)
+@pytest.mark.parametrize("tokenizer_name", TOKENIZERS)
+@pytest.mark.parametrize("skip_special_tokens", [True])
+def test_decode_prompt_logprobs(complete_sequence: str,
+                                complete_sequence_token_ids: List[int],
+                                detokenizer: Detokenizer,
+                                skip_special_tokens: bool):
+    """Verify Detokenizer decodes prompt logprobs correctly."""
+    sampling_params = SamplingParams(skip_special_tokens=skip_special_tokens,
+                                     prompt_logprobs=1)
+
+    # Run sequentially.
+    seq = create_sequence(complete_sequence_token_ids)
+    seq_group = SequenceGroup(request_id="1",
+                              seqs=[seq],
+                              sampling_params=sampling_params,
+                              arrival_time=0.0)
+    dummy_logprobs = create_dummy_logprobs(complete_sequence_token_ids)
+    detokenizer.decode_prompt_logprobs_inplace(seq_group, dummy_logprobs)
+    decoded_prompt_logprobs = dummy_logprobs
+
+    if skip_special_tokens:
+        # Text for logprobs for the chosen token should be the same as the
+        # prompt text. Note that this will only be true if we skip
+        # special tokens.
+        assert complete_sequence == "".join([
+            logprobs[token_id].decoded_token for token_id, logprobs in zip(
+                complete_sequence_token_ids, decoded_prompt_logprobs)
+        ])
+        assert complete_sequence != "".join([
+            logprobs[token_id + 1].decoded_token for token_id, logprobs in zip(
+                complete_sequence_token_ids, decoded_prompt_logprobs)
+        ])

vllm/engine/llm_engine.py

@@ -1,5 +1,5 @@
 import time
-from typing import Dict, Iterable, List, Optional, Tuple, Type, Union
+from typing import Iterable, List, Optional, Tuple, Type, Union
 
 from transformers import PreTrainedTokenizer
 
@@ -15,11 +15,11 @@ from vllm.engine.ray_utils import initialize_ray_cluster
 from vllm.logger import init_logger
 from vllm.outputs import RequestOutput
 from vllm.sampling_params import SamplingParams
-from vllm.sequence import (Logprob, SamplerOutput, Sequence, SequenceGroup,
+from vllm.sequence import (SamplerOutput, Sequence, SequenceGroup,
                            SequenceGroupOutput, SequenceOutput, SequenceStatus)
-from vllm.transformers_utils.tokenizer import detokenize_incrementally
 from vllm.transformers_utils.tokenizer_group import (BaseTokenizerGroup,
                                                      get_tokenizer_group)
+from vllm.transformers_utils.detokenizer import Detokenizer
 from vllm.utils import Counter
 
 logger = init_logger(__name__)
@@ -97,6 +97,7 @@ class LLMEngine:
         self._verify_args()
 
         self._init_tokenizer()
+        self.detokenizer = Detokenizer(self.tokenizer)
         self.seq_counter = Counter()
 
         self.model_executor = executor_class(model_config, cache_config,
@@ -153,7 +154,7 @@ class LLMEngine:
         raise RuntimeError("LLMEngine should not be pickled!")
 
     def get_tokenizer(self) -> "PreTrainedTokenizer":
-        return self.tokenizer.get_lora_tokenizer()
+        return self.tokenizer.get_lora_tokenizer(None)
 
     def get_tokenizer_for_seq(self,
                               sequence: Sequence) -> "PreTrainedTokenizer":
@@ -370,13 +371,8 @@ class LLMEngine:
         # Process prompt logprobs
         prompt_logprobs = outputs.prompt_logprobs
         if prompt_logprobs is not None:
-            # We can pick any sequence for the prompt.
+            self.detokenizer.decode_prompt_logprobs_inplace(
-            seq = next(iter(seq_group.seqs_dict.values()))
+                seq_group, prompt_logprobs)
-            all_token_ids = seq.get_token_ids()
-            for i, prompt_logprobs_for_token in enumerate(prompt_logprobs):
-                self._decode_logprobs(seq, seq_group.sampling_params,
-                                      prompt_logprobs_for_token,
-                                      all_token_ids[:i])
             seq_group.prompt_logprobs = prompt_logprobs
 
         # Process samples
@@ -420,7 +416,8 @@ class LLMEngine:
             child_seqs.append((parent, parent))
 
         for seq, _ in child_seqs:
-            self._decode_sequence(seq, seq_group.sampling_params)
+            self.detokenizer.decode_sequence_inplace(seq,
+                                                     seq_group.sampling_params)
             self._check_stop(seq, seq_group.sampling_params)
 
         # Non-beam search case
@@ -713,51 +710,6 @@ class LLMEngine:
             time_e2e_requests=time_e2e_requests,
         )
 
-    def _decode_logprobs(self, seq: Sequence, prms: SamplingParams,
-                         logprobs: Dict[int, Logprob],
-                         all_input_ids: List[int]) -> None:
-        if not logprobs:
-            return
-        for token_id, sample_logprob in logprobs.items():
-            if (sample_logprob.decoded_token is None and token_id != -1):
-                all_input_ids_with_logprob = all_input_ids[:-1] + [token_id]
-                (_, new_text, prefix_offset,
-                 read_offset) = detokenize_incrementally(
-                     self.get_tokenizer_for_seq(seq),
-                     all_input_ids=all_input_ids_with_logprob,
-                     prev_tokens=seq.tokens,
-                     prefix_offset=seq.prefix_offset,
-                     read_offset=seq.read_offset,
-                     skip_special_tokens=prms.skip_special_tokens,
-                     spaces_between_special_tokens=prms.
-                     spaces_between_special_tokens,
-                 )
-                sample_logprob.decoded_token = new_text
-
-    def _decode_sequence(self, seq: Sequence, prms: SamplingParams) -> None:
-        """Decodes the new token for a sequence."""
-        all_input_ids = seq.get_token_ids()
-        self._decode_logprobs(seq, prms, seq.output_logprobs[-1],
-                              all_input_ids)
-
-        (new_tokens, new_output_text, prefix_offset,
-         read_offset) = detokenize_incrementally(
-             self.get_tokenizer_for_seq(seq),
-             all_input_ids=all_input_ids,
-             prev_tokens=seq.tokens,
-             prefix_offset=seq.prefix_offset,
-             read_offset=seq.read_offset,
-             skip_special_tokens=prms.skip_special_tokens,
-             spaces_between_special_tokens=prms.spaces_between_special_tokens,
-         )
-        if seq.tokens is None:
-            seq.tokens = new_tokens
-        else:
-            seq.tokens.extend(new_tokens)
-        seq.prefix_offset = prefix_offset
-        seq.read_offset = read_offset
-        seq.output_text += new_output_text
-
     def _check_stop(self, seq: Sequence,
                     sampling_params: SamplingParams) -> None:
         """Stop the finished sequences."""

vllm/transformers_utils/detokenizer.py

@@ -0,0 +1,155 @@
+from typing import List, Dict, Optional
+from transformers import PreTrainedTokenizer
+from vllm.sequence import Sequence, Logprob, SequenceGroup, SamplingParams
+from vllm.transformers_utils.tokenizer import (detokenize_incrementally,
+                                               convert_prompt_ids_to_tokens)
+from vllm.transformers_utils.tokenizer_group.base_tokenizer_group import (
+    BaseTokenizerGroup)
+
+# Used eg. for marking rejected tokens in spec decoding.
+INVALID_TOKEN_ID = -1
+
+
+class Detokenizer:
+    """Provides methods to decode the output of a model into text."""
+
+    def __init__(self, tokenizer_group: BaseTokenizerGroup):
+        self.tokenizer_group = tokenizer_group
+
+    def get_tokenizer_for_seq(self,
+                              sequence: Sequence) -> "PreTrainedTokenizer":
+        """Returns the HF tokenizer to use for a given sequence."""
+        return self.tokenizer_group.get_lora_tokenizer(sequence.lora_request)
+
+    def decode_prompt_logprobs_inplace(
+            self, seq_group: SequenceGroup,
+            prompt_logprobs: List[Optional[Dict[int, Logprob]]]) -> None:
+        """Decodes the logprobs for the prompt of a sequence group.
+
+        Args:
+            seq_group: The sequence group to decode.
+            prompt_logprobs: The logprobs to decode.
+        
+        Returns:
+            The prompt logprobs with the decoded tokens.
+        """
+        prms = seq_group.sampling_params
+        # We can pick any sequence for the prompt.
+        seq = next(iter(seq_group.seqs_dict.values()))
+        # Only prompt, without the generated token.
+        all_token_ids = seq.get_token_ids()
+        prompt_token_ids = all_token_ids[:-1]
+        tokenizer = self.get_tokenizer_for_seq(seq)
+        prefix_offset = 0
+        read_offset = 0
+        next_iter_prefix_offset = 0
+        next_iter_read_offset = 0
+        next_iter_tokens = []
+        prev_tokens = None
+
+        for token_position, prompt_logprobs_for_token in enumerate(
+                prompt_logprobs):
+            if not prompt_logprobs_for_token:
+                continue
+            for token_id, sample_logprob in prompt_logprobs_for_token.items():
+                if (sample_logprob.decoded_token is None
+                        and token_id != INVALID_TOKEN_ID):
+                    prompt_token_ids_with_token = (
+                        prompt_token_ids[:token_position] + [token_id])
+                    (new_tokens, new_text, new_prefix_offset,
+                     new_read_offset) = detokenize_incrementally(
+                         tokenizer=tokenizer,
+                         all_input_ids=prompt_token_ids_with_token,
+                         prev_tokens=prev_tokens,
+                         prefix_offset=prefix_offset,
+                         read_offset=read_offset,
+                         skip_special_tokens=prms.skip_special_tokens,
+                         spaces_between_special_tokens=prms.
+                         spaces_between_special_tokens,
+                     )
+
+                    sample_logprob.decoded_token = new_text
+
+                    # Use the offsets & prev tokens corresponding to
+                    # real tokens to ensure detokenization is consistent
+                    # actual with prompt.
+                    if token_id == all_token_ids[token_position]:
+                        next_iter_prefix_offset = new_prefix_offset
+                        next_iter_read_offset = new_read_offset
+                        next_iter_tokens = new_tokens
+
+            # Advance to the next token position.
+            prefix_offset = next_iter_prefix_offset
+            read_offset = next_iter_read_offset
+            if prev_tokens is None:
+                prev_tokens = next_iter_tokens
+            else:
+                prev_tokens.extend(next_iter_tokens)
+
+    def decode_sequence_inplace(self, seq: Sequence,
+                                prms: SamplingParams) -> None:
+        """Decodes the new token for a sequence. In-place operation.
+
+        Args:
+            seq: The sequence to decode.
+            prms: The sampling parameters used to generate the sequence.
+        """
+        all_input_ids = seq.get_token_ids()
+        token_id_generated_this_iteration = all_input_ids[-1]
+        tokenizer = self.get_tokenizer_for_seq(seq)
+
+        # Convert prompt token IDs to tokens if necessary.
+        # Do it here so that we don't have to repeat this
+        # computation for each logprob.
+        if seq.tokens is None:
+            (seq.tokens, seq.prefix_offset,
+             seq.read_offset) = convert_prompt_ids_to_tokens(
+                 tokenizer=tokenizer,
+                 prompt_ids=all_input_ids[:-1],
+                 skip_special_tokens=prms.skip_special_tokens,
+             )
+
+        (new_tokens, new_decoded_token_text, prefix_offset,
+         read_offset) = detokenize_incrementally(
+             tokenizer=tokenizer,
+             all_input_ids=all_input_ids,
+             prev_tokens=seq.tokens,
+             prefix_offset=seq.prefix_offset,
+             read_offset=seq.read_offset,
+             skip_special_tokens=prms.skip_special_tokens,
+             spaces_between_special_tokens=prms.spaces_between_special_tokens,
+         )
+
+        # Decode logprobs
+        logprobs = seq.output_logprobs[-1]
+        if logprobs:
+            previous_tokens = all_input_ids[:-1]
+            for token_id, sample_logprob in logprobs.items():
+                # If the token was generated this iteration,
+                # use the provided text.
+                if token_id == token_id_generated_this_iteration:
+                    sample_logprob.decoded_token = new_decoded_token_text
+                    continue
+
+                if (sample_logprob.decoded_token is None
+                        and token_id != INVALID_TOKEN_ID):
+                    all_input_ids_with_logprob = previous_tokens + [token_id]
+                    (_, new_text, _, _) = detokenize_incrementally(
+                        tokenizer=tokenizer,
+                        all_input_ids=all_input_ids_with_logprob,
+                        prev_tokens=seq.tokens,
+                        prefix_offset=seq.prefix_offset,
+                        read_offset=seq.read_offset,
+                        skip_special_tokens=prms.skip_special_tokens,
+                        spaces_between_special_tokens=prms.
+                        spaces_between_special_tokens,
+                    )
+                    sample_logprob.decoded_token = new_text
+
+        if seq.tokens is None:
+            seq.tokens = new_tokens
+        else:
+            seq.tokens.extend(new_tokens)
+        seq.prefix_offset = prefix_offset
+        seq.read_offset = read_offset
+        seq.output_text += new_decoded_token_text

vllm/transformers_utils/tokenizer.py

@@ -158,6 +158,34 @@ def _convert_tokens_to_string_with_added_encoders(
         return "".join(sub_texts)
 
 
+# 5 is an arbitrary value that should work for all
+# tokenizers (bigger = more conservative).
+INITIAL_INCREMENTAL_DETOKENIZATION_OFFSET = 5
+
+
+def convert_prompt_ids_to_tokens(
+    tokenizer: Union[PreTrainedTokenizer, PreTrainedTokenizerFast],
+    prompt_ids: List[int],
+    skip_special_tokens: bool = False,
+) -> Tuple[List[str], int, int]:
+    """Converts the prompt ids to tokens and returns the tokens and offsets
+    for incremental detokenization.
+
+    Note that not all tokens are converted to strings. Only the tokens that
+    are necessary for incremental detokenization are converted to strings.
+    """
+    # Offset a little more in case we have special tokens.
+    prefix_offset = max(
+        len(prompt_ids) - INITIAL_INCREMENTAL_DETOKENIZATION_OFFSET - 2, 0)
+    # We do not need to convert the whole prompt to tokens.
+    new_tokens = tokenizer.convert_ids_to_tokens(
+        prompt_ids[prefix_offset:], skip_special_tokens=skip_special_tokens)
+    prefix_offset = max(
+        len(new_tokens) - INITIAL_INCREMENTAL_DETOKENIZATION_OFFSET, 0)
+    read_offset = len(new_tokens)
+    return new_tokens, prefix_offset, read_offset
+
+
 # Based on
 # https://github.com/huggingface/text-generation-inference/blob/v0.9.4/server/text_generation_server/models/model.py#L62C9-L62C15
 # under Apache 2.0 license
@@ -165,31 +193,53 @@ def detokenize_incrementally(
     tokenizer: Union[PreTrainedTokenizer, PreTrainedTokenizerFast],
     all_input_ids: List[int],
     prev_tokens: Optional[List[str]],
-    prefix_offset: int = 0,
+    prefix_offset: int,
-    read_offset: int = 0,
+    read_offset: int,
     skip_special_tokens: bool = False,
     spaces_between_special_tokens: bool = True,
 ) -> Tuple[List[str], str, int, int]:
+    """Detokenizes the input ids incrementally and returns the new tokens
+    and the new text.
+
+    If `prev_tokens` is None, this function will convert the input ids to
+    tokens and return the tokens and the new text. Otherwise, it will return the
+    new tokens and the new text.
+
+    This function will also return the new prefix offset and the new read
+    offset to be used in the next iteration.
+
+    The offsets are necessary to defeat cleanup algorithms in the decode which
+    decide to add a space or not depending on the surrounding ids.
+
+    Args:
+        tokenizer: The tokenizer to use.
+        all_input_ids: The input ids. The last id is the new token id.
+        prev_tokens: The previous tokens. If None, this function will convert
+            the input ids to tokens and return the tokens and the new text.
+        prefix_offset: The prefix offset.
+        read_offset: The read offset.
+        skip_special_tokens: Whether to skip special tokens.
+        spaces_between_special_tokens: Whether to add spaces between special
+            tokens.
+    """
     new_token_id = all_input_ids[-1]
     # This is the first iteration for this sequence
-    if prev_tokens is None:
+    is_first_iter = prev_tokens is None
-        new_tokens = tokenizer.convert_ids_to_tokens(
+    if is_first_iter:
-            all_input_ids, skip_special_tokens=skip_special_tokens)
+        (prev_tokens, prefix_offset,
-        output_tokens = new_tokens
+         read_offset) = convert_prompt_ids_to_tokens(
-        # 5 is an arbitrary value that should work for all
+             tokenizer,
-        # tokenizers (bigger = more conservative).
+             all_input_ids[:-1],
-        # Subtract 1 extra to account for the generated token.
+             skip_special_tokens=skip_special_tokens)
-        prefix_offset = max(len(output_tokens) - 6, 0)
+
-        # If the first new token is a special token, we can't skip 1 extra token
+    # Put new_token_id in a list so skip_special_tokens is respected
-        if skip_special_tokens and new_token_id in tokenizer.all_special_ids:
+    new_tokens = tokenizer.convert_ids_to_tokens(
-            read_offset = max(len(output_tokens), 0)
+        [new_token_id], skip_special_tokens=skip_special_tokens)
-        else:
+    output_tokens = prev_tokens + new_tokens
-            read_offset = max(len(output_tokens) - 1, 0)
+
-    else:
+    # If this is the first iteration, return all tokens.
-        # Put new_token_id in a list so skip_special_tokens is respected
+    if is_first_iter:
-        new_tokens = tokenizer.convert_ids_to_tokens(
+        new_tokens = output_tokens
-            [new_token_id], skip_special_tokens=skip_special_tokens)
-        output_tokens = prev_tokens + new_tokens
 
     # The prefix text is necessary only to defeat cleanup algorithms in
     # the decode which decide to add a space or not depending on the