Defaults are unbound if not present. The implementation selects an option's Default from the given list of Defaults using select_option/4. Option terms must have one variable. This is because select_option/4's fourth argument is a single argument. It never unifies with multiple variables even though it succeeds, e.g. select_option(a(A, B), [], Rest, 1) unifies A with 1, leaving B unbound.

There is a naming issue. What to call the incoming list of Option arguments and the Options argument with which the Option terms unify? One possibility: name the Options argument RestOptions0 since they represent the initial set of RestOptions from which Options select. This clashes with select_option/4's naming convention since Options is the argument name for RestOptions0's role in the option-selection process. Nevertheless, this version follows this renamed argument convention.