code: def _replace_arg(arg, tvars, args):
      
  
      if tvars is None:
          tvars = []
      if hasattr(arg, '_subs_tree') and isinstance(arg, (GenericMeta, _TypingBase)):
          return arg._subs_tree(tvars, args)
      if isinstance(arg, TypeVar):
          for i, tvar in enumerate(tvars):
              if arg == tvar:
                  return args[i]
      return arg
docs: An internal helper function: replace arg if it is a type variable
      found in tvars with corresponding substitution from args or
      with corresponding substitution sub-tree if arg is a generic type.