factorial.md

# Define a procedure, factorial, that takes a natural number as its input, and
# returns the number of ways to arrange the input number of items.

def factorial(n):
    if n == 0 :
        return 1
    else:
        return n * factorial(n-1)


print factorial(0)
#>>> 1

print factorial(5)
#>>> 120

print factorial(10)
#>>> 3628800