Return the independent variables in a symfun.
For a @sym, this always returns the empty sym, but subclasses like @symfun do something more interesting.
Example:
syms x y f = 2*x*y; argnames(f) ⇒ (sym) [] (empty 0×0 matrix)
See also: @symfun/argnames, symvar, findsym, findsymbols.
Package: symbolic