This approach can only be applied to
primitive deterministic
procedures.
While unification in the head and the primitive body code takes place, only a link to the next alternative clause is needed as backtrack information because no heap variables are bound, no non-primitive goal in the body will be called, and no side effects will occur. On the other hand, after successful unification in the head and the primitives no backtracking in this procedure is possible because the only possible matching clause is selected.
This reduces the code space requirements at run time, but good global analyzing methods are needed to detect primitive deterministic procedures.