Retrieve the function specified by the given object.
f Any object referencing a function.
func The function object if one is referenced by the input, and null otherwise.

If the input is a string, an attempt is made to load this function from the files in the current path. If a function with this name is found, it is the output of the function. Otherwise, the an attempt is made to load the string as an inline function.

If the input is a function object, the input is returned.

In all other cases, an error condition is created that the input is not a function.
Builtin function:sin
inline(x) = x+2