Sum range

Define a function with type:

sumrange : int -> int -> int

such that sumrange a b sums the numbers in the range [a,b]. If the range is empty, then the result is zero.

Here are some unit tests:

assert (sumrange 0 1 = 1);;
assert (sumrange 1 3 = 6);;
assert (sumrange 3 2 = 0);;