test for factorial
This function will return true if expr is a factorial, and false otherwise. For more information about factorials, see factorial.
An expression of the type n!, where n is an integer, is not of type `!`, since its value is calculated before the call to the type function is executed.
Note that the factorial function is both of type function and type `!`. In the function call, it is important that the exclamation mark, !, be enclosed in quotes. Missing quotes will cause a syntax error.
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