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Mr. Dickinson thank you for doing this. I do not know how to help with 
a patch. If it helps, here is the code I use in python:
def factorial(n, _known=[1]):
 assert isinstance(n, int), "Need an integer. This isn't a gamma"
 assert n >= 0, "Sorry, can't factorilize a negative"
 assert n < 1000, "No way! That's too large"
 try:
 return _known[n]
 except IndexError:
 pass
 for i in range(len(_known), n+1):
 _known.append(_known[-1] * i)
 return _known[n]
When the assertions are turned-off, this runs pretty fast.