code: def parse_type_system_definition(lexer: Lexer) -> TypeSystemDefinitionNode:
      
      # Many definitions begin with a description and require a lookahead.
      keyword_token = lexer.lookahead() if peek_description(lexer) else lexer.token
      func = _parse_type_system_definition_functions.get(cast(str, keyword_token.value))
      if func:
          return func(lexer)
      raise unexpected(lexer, keyword_token)
docs: TypeSystemDefinition