Availability:built-in
nb_getval(+Name, 
-Value)The nb_getval/2 
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 existence_error(variable, 
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.