corrected problem with auto_tac: now uses a variant of depth_tac that avoids
interference of the simplifier with dup_step_tac
Arith = FOL +
classes arith < term
consts "0" :: "'a::arith" ("0")
"1" :: "'a::arith" ("1")
"+" :: "['a::arith,'a] => 'a" (infixl 60)
end