NTHARG : WFFOPNTHARG is a wff operation.
Move to the nth argument of a functional application,
or to the nth disjunct, conjunct, etc.
The calling scheme for NTHARG is:
(NTHARG N GWFF)
The result is of type GWFF.
The arguments have the following meaning:
N : Number of argument (0 means the function)
GWFF : NIL
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