Extensible helper tactic for
decreasing_tactic. This handles the "base case"
reasoning after applying lexicographic order lemmas.
It can be extended by adding more macro definitions, e.g.
macro_rules | `(tactic| decreasing_trivial) => `(tactic| linarith)
decreasing_tactic is called by default on well-founded recursions in order
to synthesize a proof that recursive calls decrease along the selected
well founded relation. It can be locally overridden by using
on the recursive definition, and it can also be globally extended by adding
more definitions for
which this tactic calls).