A deep dictionary is a dict in which each value is deep list: Each key has the same reprentation it would have in a standard dict, and each value is either a simple value or a deep dictionary. A simple value is represented as a list containing only one item.
Unlike a standard Tcl dictionary, a deep dictionary is suitable for communicating an ordered tree without prior agreement between the parties about the exact shape of the tree. A deep dictionary can be unambiguously merged into another deep dictionary.