docs: def ensure_context(**vars):
      """Ensures that a context is in the stack, creates one otherwise.
      """
      ctx = _context_stack.top
      stacked = False
      if not ctx:
          ctx = Context()
          stacked = True
          _context_stack.push(ctx)
      ctx.update(vars)
      try:
          yield ctx
      finally:
          if stacked:
              _context_stack.pop()