doc-src/Intro/arith.thy

cleaned sig;
removed add_defns (now in drule.ML as add_defs);
removed add_sigclass;
minor internal changes;

Arith = FOL +
classes arith < term
consts  "0"     :: "'a::arith"                  ("0")
        "1"     :: "'a::arith"                  ("1")
        "+"     :: "['a::arith,'a] => 'a"       (infixl 60)
end