docs: def execution_context_from_async(async):
      """Instantiate a new _ExecutionContext and store a reference to it in the
      global async context to make later retrieval easier.
      """
      local_context = _local.get_local_context()
  
      if local_context._executing_async_context:
          raise errors.ContextExistsError
  
      execution_context = _ExecutionContext(async)
      local_context._executing_async_context = execution_context
      return execution_context