docs: def get_aliases(lines):
      """Get the type aliases in the source.
  
      :sig: (Sequence[str]) -> Dict[str, str]
      :param lines: Lines of the source code.
      :return: Aliases and their their definitions.
      """
      aliases = {}
      for line in lines:
          line = line.strip()
          if len(line) > 0 and line.startswith(SIG_ALIAS):
              _, content = line.split(SIG_ALIAS)
              alias, signature = [t.strip() for t in content.split("=")]
              aliases[alias] = signature
      return aliases