added cproj', and therefore extended prj
tried to fix polymorphism problem for take_defs. A full solution will require
significant changes.
Arith = FOL +
classes arith < term
consts "0" :: "'a::arith" ("0")
"1" :: "'a::arith" ("1")
"+" :: "['a::arith,'a] => 'a" (infixl 60)
end