moved recursion combinator to HOL/Library/Wfrec.thy -- it is so fundamental and well-known that it should survive recdef
Arith = FOL +
classes arith < term
consts "0" :: "'a::arith" ("0")
"1" :: "'a::arith" ("1")
"+" :: "['a::arith,'a] => 'a" (infixl 60)
end