Pure/tactic/compose_inst_tac: when catching exception THM, prints the
message before failing!! This reports the reason for failure in cases like
by (res_inst_tac [("P", "?Q(a)")] mp 1);
in which ?Q appears in mp with a different type.
Bool = FOL +
types bool 0
arities bool :: term
consts tt,ff :: "bool"
end