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 _ for 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.

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.