"I think even I could code the little trading computer with something like "if bet>$1 trillion, deny authorization." Or, ok, these things are complicated, how about if "Probability of loss>$1 trillion is >.00001 then deny authorization."
From what I recall, the calculation of a "risk" value for an individual trade isn't too bad but it gets exciting when you try to do this for a lot of trades across a complex portfolio - especially when you have to factor in other kinds of risk (e.g. forex).
I was the lead software guy and it was fascinating to work with the ex-traders who were providing the financial knowledge. A shame that the product died horribly due to litigation on contractual issues.