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Exercise 4.2
A stream of factorial numbers

Make a stream of all factorial numbers: 1 2 6 24 120 ...

Please go for a recursive definition, in the same style as used for nat-nums and fib.

As part of the solution, you may need a multiplicative stream function similar to add-streams. For that purpose make a higher-order function, combine-streams, that combines two (infinite) streams with a binary function.