predicate is a synonym for b_getval/2,
introduced for compatibility and symmetry. As most scenarios will use a
particular global variable using either non-backtrackable or
backtrackable assignment, using nb_getval/2
can be used to document that the variable is non-backtrackable. Raises
Name) if the variable does not exist. Alternatively, nb_current/2
can used to query a global variable. This version fails if the
variable does not exist rather than raising an exception.