Pure/Unify/IMPROVING "CLEANING" OF FLEX-FLEX PAIRS: Old code would refuse
to simplfy %x y.?a(x) =?= %x y.?b(f(?c(y), y)) because it found the flexible
path to y before the rigid path. New code simplifies this to
%x.?a(x) =?= %x.?d, eliminating ?a.
Pure/Unify/rigid_bound: preliminary check for rigid paths to the banned
bound variables
Pure/Unify/change_bnos: any occurrences of the banned bound variables must
now be flexible, causing abandonment of the "cleaning"