Sort search results in order of relevance, where relevance =
a) better if 0 premises for intro or 1 premise for elim/dest rules
b) better if substitution size wrt to current goal is smaller
Only applies to intro, dest, elim, and simp
(contributed by Rafal Kolanski, NICTA)
If = FOLP +
consts if :: "[o,o,o]=>o"
rules
if_def "if(P,Q,R) == P&Q | ~P&R"
end