code: def _parse_letter_version(
      letter: str, number: Union[str, bytes, SupportsInt]
  ) -> Optional[Tuple[str, int]]:
      if letter:
          # We consider there to be an implicit 0 in a pre-release if there is
          # not a numeral associated with it.
          if number is None:
              number = 0
  
          # We normalize any letters to their lower case form
          letter = letter.lower()
  
          # We consider some words to be alternate spellings of other words and
          # in those cases we want to normalize the spellings to our preferred
          # spelling.
          if letter == "alpha":
              letter = "a"
          elif letter == "beta":
              letter = "b"
          elif letter in ["c", "pre", "preview"]:
              letter = "rc"
          elif letter in ["rev", "r"]:
              letter = "post"
  
          return letter, int(number)
      if not letter and number:
          # We assume if we are given a number, but we are not given a letter
          # then this is using the implicit post release syntax (e.g. 1.0-1)
          letter = "post"
  
          return letter, int(number)
  
      return None
  
imports: import collections
  import itertools
  import re
  from typing import Any, Callable, Optional, SupportsInt, Tuple, Union
  from ._structures import Infinity, InfinityType, NegativeInfinity, NegativeInfinityType
label_desc: Use image node_id 2 for calling a global function with example usage: _parse_letter_version(letter, number) and returns: None, letter,  int, letter,  int