-- tail recursion

def sum' := \x \acc if x = 0 then acc else sum' (x - 1) (x + acc);
def sum := \x sum' x 0;

def fact' := \x \acc if x = 0 then acc else fact' (x - 1) (acc * x);
def fact := \x fact' x 1;

def writeLn := \x write x >> write "\n";

writeLn (sum 10000) >>
writeLn (fact 5) >>
writeLn (fact 10) >>
writeLn (fact 12)