The motivation is to reduce the number of instances generated by ematching.
For example, given
inv_inv: forall a, (a⁻¹)⁻¹ = a
the new heuristic uses ((a⁻¹)⁻¹) as the pattern.
This matches the intuition that inv_inv should be used a simplification
rule.
The default pattern inference procedure would use (a⁻¹). This is bad
because it generates an infinite chain of instances whenever there is a
term (a⁻¹) in the proof state.
By using (a⁻¹), we get
(a⁻¹)⁻¹ = a
Now that we have (a⁻¹)⁻¹, we can match again and generate
((a⁻¹)⁻¹)⁻¹ = a⁻¹
and so on
