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Undefined values: Difference between revisions
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=={{header|Prolog}}==
Prolog has two predicates to know if a variable is instancied or not : '''var/1''' and '''nonvar/1''' <br>
<pre>?- var(Y).
true.
?- X = 4, var(X).
false.
?- nonvar(Y).
false.
?- X = 4, nonvar(X).
X = 4.
</pre>
=={{header|PureBasic}}==
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