reverted e7a80c6752c9 -- there's not much point in putting a diagnosis tool (as opposed to a proof method) in Plain, but more importantly Sledgehammer must be in Main to use SMT solvers
Arith = FOL +
classes arith < term
consts "0" :: "'a::arith" ("0")
"1" :: "'a::arith" ("1")
"+" :: "['a::arith,'a] => 'a" (infixl 60)
end