text: # coding=utf-8
  # Copyright 2021 The Google Research Authors.
  #
  # Licensed under the Apache License, Version 2.0 (the "License");
  # you may not use this file except in compliance with the License.
  # You may obtain a copy of the License at
  #
  #     http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS,
  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  # See the License for the specific language governing permissions and
  # limitations under the License.
  
  """Constants used by Felix models."""
  
  # Edit operations.
  KEEP = 'KEEP'
  DELETE = 'DELETE'
  PAD_TAG = 'PAD'
  
  
  # Special tokens.
  PAD = '[PAD]'
  CLS = '[CLS]'
  SEP = '[SEP]'
  MASK = '[MASK]'
  
  # Special tokens that indicate the start and end of a span of deleted tokens.
  DELETE_SPAN_START = '[unused1]'
  DELETE_SPAN_END = '[unused2]'
  
  # For filtering out input tokens which are not used.
  DELETED_TAGS = frozenset([DELETE, PAD_TAG, PAD])
  