Aside: Instantiation Terminology

We say that a variable is
    instantiated        if it is bound to a value.
                        E.g.: X = 1
                              X = [a,b,c]
    uninstantiated      otherwise.
                        E.g.: X = Y (assuming Y is also uninstantiated)

An instantiated variable can be
    partially instantiated      if it is bound to a structure that
                                contains uninstantiated variables.
                                E.g.: X = [a,b,Y]
                                      X = [Y|Z]
    fully instantiated          otherwise.
       (or "ground")            E.g.: X = [a,b,c]
                                      X = 1
                                      X = [d|f]

These terms are often used to specify the preconditions of a predicate.
E.g.:
% selectionsort(List,Sorted) holds if Sorted is List sorted.
% Precondition: List is a fully instantiated list of numbers.