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

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

fn if {
  [True]  => fn { [x] => fn { [_] => x } };
  [False] => fn { [_] => fn { [y] => y } };
};
fn 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"];