FUNCTION binom,n,k,p

If k LT 0 THEN Message,'Cannot have negative events.'
If n LT 0 THEN Message,'Cannot have negative trials.'
If p LT 0 OR p GT 1 THEN Message,'p not in interval [0,1]'
If k EQ n THEN Return,Binomial(n,n,p)
If k LT n THEN Return,Binomial(k,n,p)-Binomial(k+1,n,p)
END