Bump Set for Hashable.
Note to self.
Add a binary tree implementation.
Fix up some def’ns.
Let parse trees hash themselves.
Replace binary trees with parse trees.
Bump Set for unindexed subscripting.
ParseTree can do minimizing constructions.
ParseTree is Printable.
Require the prefix for the sequence initializer.
ParseTree can be union’d.
ParseTree can be counted.
Parse forests are parse trees, not sets.

Parse trees have choice nodes, allowing ambiguity to be represented
correctly.
Style.
Documentation.
More documentation.
Updates for syntax changes.
Remove unused equality on languages.
Non-final classes can have non-fixed layout now.
Documentation.