def
choose( n, k ) | k < 0 or k > n = 0
choose( n, 0 ) = 1
choose( n, n ) = 1
choose( n, k ) = product( [(n - i)/(i + 1) | i <- 0:min( k, n - k )] )
println( choose(5, 3) )
println( choose(60, 30) )