This is a warning to help you with two common mistakes:

• Spelling mistakes in variables
• Forget to use/bind a variable It indicates that there is one or more variable in the clause that appears only once. This is never necessary as the first appearance of a variable always succeeds with a successful binding. If this binding is not used anywhere, nothing happens. You may compare it to gcc's warning "statement has no effect".

  But, what else do I place there? Prolog has the anonymous variable named _ this purpose. This variable has `no name', unifies to anything without any effect. If _ appears multiple times in the same term, they refer to distinct variables.

  But, how do I document what I ignore? Prolog systems won't complain on variables that start with an underscore. Thus, the variable _Country won't be reported if it is singleton. Note however that where two appearances of _ are distinct variables, two appearances of _Country are not: they are the same variable. Trying to load a clause holding two times _Country results in this warning:

  Warning:    Singleton-marked variable appears more than once: _Country

  But, the program I received has tons. What now? For this emergency there is the directive style_check/1. The code below compiles silently.

  :- style_check(-singleton).
  
  better('SWI-Prolog', AnyOtherProlog).

  Note: changes to the style_check/1 options are reverted at the end of the file the directive appears in. See also Syntax Notes in the reference manual.

Singleton variable in branch

The above singleton warnings are reported by many Prolog systems. They are purely syntactical. In addition, SWI-Prolog also provides a semantical analysis that validates that the a variable that appears twice actually enforces unification. Consider the code below. Here, X is not a singleton, but it as they two statements are in different branches, X does not force the same X in p(X) and q(X).

t :-
    (   p(X)
    ;   q(X)
    ).

Compiling this results in this warning

Warning: Singleton variable in branch: X

Note that this only applies to branches. Calls to meta predicates may also cause that two variables do not actually share. This is not reported. For example, the code below is semantically equivalent to the code above, but is (currently) silently compiled.

or(A,B) :- (A;B).
    t :- or(p(X), q(X).

Suppressing all warnings on a variable

SWI-Prolog never warns on a variable where the name starts with an _, followed by a digit. So, _0Country will never lead to a warning, regardless of flags, how often this name appears in a clause and the semantic relationship between these occurrences.