remove old implementation
warm up: reachability tests

Returning a simple True or False instead of a transformation between the
two Vars.
extra case: reachable after weakening
MultiSet.Eq
fix swapReachable bug

(Pair x y) should be reachable from (Pair x y).
simplify {exchange,weaken}Reachable via MultiSet
document a flaw in weakenReachable
extra case: reachable after duplicating variables