docs: def _format_stack(self, stack, current=None):
          """Prettifies a scope stack for use in error messages.
  
          :param list(str) stack: List of scopes.
          :param str current: The current scope. If specified, will be appended
              onto the stack before formatting.
          :returns: str
          """
          if current is not None:
              stack = stack + [current]
          if len(stack) > 1:
              prefix = os.path.commonprefix(stack)
              if prefix.endswith('/'):
                  prefix = prefix[:-1]
              stack = [scope[len(prefix):] for scope in stack]
          return ' => '.join(stack)