* The code in defaultcontext/stack.py is based on a _DefaultStack implementation from the TensorFlow project:
  https://github.com/tensorflow/tensorflow/blob/0b723590631432584d0761c03285eabb55116c6d/tensorflow/python/framework/ops.py#L3798
  The TensorFlow project is licensed under Apache 2.0 license. The copy of the license included in the defaultcontext/stack.py.
