Module termops 

Term-introspection builtins: functor/3, arg/3, =../2 (univ), copy_term/2.

Tag decisions:

• functor(T, '.', 2) and T =.. ['.', a, b] both build a STR (functor “.”, arity 2), NOT a list cell.
• decomposing a TAG_LST yields functor . / arity 2 and args [Head, Tail] (univ produces ['.', H, T]).
• errors are structured balls (see the captured strings in the unit tests).

Functions

plg_rt_b_arg_3

arg/3: the N-th argument of a compound. 1 = success, 0 = fail/error.

plg_rt_b_copy_term_2

copy_term/2: a fresh copy of orig with consistent renamed vars.

plg_rt_b_functor_3

functor/3: decompose or construct. 1 = success, 0 = failure/error.

plg_rt_b_univ_2

=../2 (univ): decompose into / build from a list. 1 = success.