isZero := { Z => True; <S,_> => False };
add := { Z     => { y => y }
       ; <S,x> => { y => add.x.<S,y> } };
incr := add.<S,Z>;

two  := <S,<S,Z>>;
four := <S,<S,<S,<S,Z>>>>;

if := { True  => { x => { _ => x } }
      ; False => { _ => { y => y } }
      };
minusOne := { Z => Z; <S,x> => x };

puts "S is " S;
puts "Z is " Z;

puts "True is " True;
puts "False is " False;

puts "Is 0 zero?";
puts isZero.Z;

puts "Is 4 zero?";
puts isZero.four;

puts "2 + 4 = ";
puts add.four.two;

puts "Is zero zero?";
puts if.(isZero.Z)."yes"."no";

puts "Is 0+1 zero?";
puts if.(isZero.(incr.Z))."yes"."no";