take into consideration whether a fact is an "intro"/"elim"/"simp" rule as an additional factor influencing the relevance filter
Arith = FOL +
classes arith < term
consts "0" :: "'a::arith" ("0")
"1" :: "'a::arith" ("1")
"+" :: "['a::arith,'a] => 'a" (infixl 60)
end