Metaterms are treated like normal variables, except that their attributes are copied as specified by the corresponding copy_term handler.
The effect of copy_term/2 is almost the same as of the sequence
setval(reserved_name, OldTerm), getval(reserved_name, NewTerm)or a similar application of record/erase or assert/retract. However, copy_term/2 is faster, handles attributed variables more sensibly and is also more space efficient since it does not physically copy ground subterms.
Note that when the structure of the term to be copied is known, then it is more efficient to use specialised unification code or a combination of functor/3 and arg/3 to do the job.
Success: copy_term(a, C). (gives C=a). copy_term(s(X,a,Y,X), C). (gives C=s(_g1, a, _g2, _g1)). copy_term([X,2|Y], C). (gives C=[_g1, 2| _g2]). copy_term(X, C). copy_term(X, s(1,2,3)). Fail: copy_term(s(X,X), s(3,4)).