def fac(n):
"""Factorial n!. Returns integer."""
_assert(isinstance(n, int), '`n` must be int')
_assert(n >= 0, '`n` must be >= 0')
if n == 1 or n == 0:
return 1
return n * fac(n-1)
def binom(n, k):
"""Binomial coefficient ("n over k").
fac(n)/(fac(k)*fac(n-k))
"""
_assert(n >= k >= 0, 'illegal input, only n >= k >= 0')
return fac(n)/(fac(k)*fac(n-k))
