A mapping that identifies definitionally equal expressions.
We implement it as a mapping from HeadIndex to AssocList Expr α.
Remark: this map may be quite inefficient if there are many HeadIndex collisions.
A mapping that identifies definitionally equal expressions.
We implement it as a mapping from HeadIndex to AssocList Expr α.
Remark: this map may be quite inefficient if there are many HeadIndex collisions.