code: def is_transition_allowed(
      constraint_type: str,
      from_tag: str,
      from_entity: str,
      to_tag: str,
      to_entity: str,
  ):
      """
      Given a constraint type and strings `from_tag` and `to_tag` that
      represent the origin and destination of the transition, return whether
      the transition is allowed under the given constraint type.
  
      # Parameters
  
      constraint_type : `str`, required
          Indicates which constraint to apply. Current choices are
          "BIO", "IOB1", "BIOUL", and "BMES".
      from_tag : `str`, required
          The tag that the transition originates from. For example, if the
          label is `I-PER`, the `from_tag` is `I`.
      from_entity : `str`, required
          The entity corresponding to the `from_tag`. For example, if the
          label is `I-PER`, the `from_entity` is `PER`.
      to_tag : `str`, required
          The tag that the transition leads to. For example, if the
          label is `I-PER`, the `to_tag` is `I`.
      to_entity : `str`, required
          The entity corresponding to the `to_tag`. For example, if the
          label is `I-PER`, the `to_entity` is `PER`.
  
      # Returns
  
      `bool`
          Whether the transition is allowed under the given `constraint_type`.
      """
  
      if to_tag == "START" or from_tag == "END":
          # Cannot transition into START or from END
          return False
  
      if constraint_type == "BIOUL":
          if from_tag == "START":
              return to_tag in ("O", "B", "U")
          if to_tag == "END":
              return from_tag in ("O", "L", "U")
          return any(
              [
                  # O can transition to O, B-* or U-*
                  # L-x can transition to O, B-*, or U-*
                  # U-x can transition to O, B-*, or U-*
                  from_tag in ("O", "L", "U")
                  and to_tag in ("O", "B", "U"),
                  # B-x can only transition to I-x or L-x
                  # I-x can only transition to I-x or L-x
                  from_tag in ("B", "I")
                  and to_tag in ("I", "L")
                  and from_entity == to_entity,
              ]
          )
      elif constraint_type == "BIO":
          if from_tag == "START":
              return to_tag in ("O", "B")
          if to_tag == "END":
              return from_tag in ("O", "B", "I")
          return any(
              [
                  # Can always transition to O or B-x
                  to_tag in ("O", "B"),
                  # Can only transition to I-x from B-x or I-x
                  to_tag == "I"
                  and from_tag in ("B", "I")
                  and from_entity == to_entity,
              ]
          )
      elif constraint_type == "IOB1":
          if from_tag == "START":
              return to_tag in ("O", "I")
          if to_tag == "END":
              return from_tag in ("O", "B", "I")
          return any(
              [
                  # Can always transition to O or I-x
                  to_tag in ("O", "I"),
                  # Can only transition to B-x from B-x or I-x, where
                  # x is the same tag.
                  to_tag == "B"
                  and from_tag in ("B", "I")
                  and from_entity == to_entity,
              ]
          )
      elif constraint_type == "BMES":
          if from_tag == "START":
              return to_tag in ("B", "S")
          if to_tag == "END":
              return from_tag in ("E", "S")
          return any(
              [
                  # Can only transition to B or S from E or S.
                  to_tag in ("B", "S") and from_tag in ("E", "S"),
                  # Can only transition to M-x from B-x, where
                  # x is the same tag.
                  to_tag == "M"
                  and from_tag in ("B", "M")
                  and from_entity == to_entity,
                  # Can only transition to E-x from B-x or M-x, where
                  # x is the same tag.
                  to_tag == "E"
                  and from_tag in ("B", "M")
                  and from_entity == to_entity,
              ]
          )
      else:
          raise ConfigurationError(
              f"Unknown constraint type: {constraint_type}"
          )
  
file_path: allennlp/allennlp/modules/conditional_random_field/conditional_random_field.py
imports: from typing import List, Tuple, Dict, Union
  import torch
  from allennlp.common.checks import ConfigurationError
  import allennlp.nn.util
label_desc: Use image node_id 2 for calling a global function with example usage: is_transition_allowed(constraint_type, from_tag, from_entity, to_tag, to_entity) and returns: False, any, unknown, unknown, any, unknown, unknown, any, unknown, unknown, any, unknown, unknown