-- NatNums.russell
        open <*
            bool: safetype ~ { t:[] },
        num: safetype ~ { pred: num },
        equal: func[ [a:num,b:num] -> bool] ~
        lambda x:[a:num,b:num] in
        if
        x.a?pred => if x.b?pred => equal <a~x.a.pred,b~x.b.pred> else <> fi
        else => if x.b?pred => <> else <t~<>> fi
        fi
        ni ,
        plus: func[ [a:num,b:num] -> num] ~
        lambda x:[a:num,b:num] in
        if
        x.a?pred => < pred ~ plus<a~x.a.pred,b~x.b>>
        else => x.b
        fi
        ni ,
        minus: func[ [a:num,b:num] -> num] ~
        lambda x:[a:num,b:num] in
        if
        x.b?pred => if x.a?pred => minus <a~x.a.pred,b~x.b.pred> else zero<> fi
        else => x.a
        fi
        ni ,
        zero: func[ []->num ] ~
        lambda a:[] in <> ni ,
        succ: func[ a:num->num ] ~
        lambda a:num in <pred~a> ni ,
        pred: func[ a:num->num ] ~
        lambda a:num in if a?pred => a.pred else <> fi ni
        *> in
        open ref[num] in
        open < n:loc~new<> > in
        assign< lhs~n, rhs~succ(zero<>) > ;
        assign< lhs~n, rhs~plus< a~valOf n, b~valOf n >> ;
        assign< lhs~n, rhs~plus< a~valOf n, b~valOf n >>
        ni
        ni
        ni