Pure: fixed idt/idts vs. pttrn/pttrns syntactic categories;
HOLCF: fixed LAM <x,y,zs>.b syntax (may break some unusual cases);
Arith = FOL +
classes arith < term
consts "0" :: "'a::arith" ("0")
"1" :: "'a::arith" ("1")
"+" :: "['a::arith,'a] => 'a" (infixl 60)
end