code: def parent(self, parent):
          
          self._parent = parent
          if parent:
              pctx = dict((x, getattr(parent, x)) for x in parent.context_keys)
              self.inject_context(pctx)
              self.depth = parent.depth + 1
              for command in self.subcommands.values():
                  command.parent = self  # bump.
          else:
              self.depth = 0
docs: Copy context from the parent into this instance as well as
          adjusting or depth value to indicate where we exist in a command
          tree.