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)
List = FOL +
types list 1
arities list :: (term)term
consts Nil :: "'a list"
Cons :: "['a, 'a list] => 'a list"
end