This modification was suggested by @kha.
TODO:
- Use `simp [-f]` instead of `simp without f`
- Allow users to remove hypothesis from `*`. Example: `simp [*, -h]`
for simplify using all hypotheses but `h`.
rel_tac is a tactic used for synthesizing a well founded relation.
The default implementation just uses type class resolution.
More sophisticated strategies may need to access the set of recursive
equations. This commit addresses this need.