class_docstr: Interface for all envelopes.
  Derive from this class, replacing the abstract methods
code: def __init__(self, handle_fn):
      self._handle_fn = handle_fn
      self.context = None
  
imports: from abc import ABC, abstractmethod
label_desc: Use image node_id 1 to create a new BaseEnvelope object from inherited base classes: ABC with example: obj = BaseEnvelope(handle_fn)