Index: lib/TWiki/If.pm
===================================================================
--- lib/TWiki/If.pm	(revision 13708)
+++ lib/TWiki/If.pm	(working copy)
@@ -251,6 +251,21 @@
             return 0;
         },
     },
+    {
+        name => 'isempty',
+        prec => 5,
+        arity => 1, # unary
+        casematters => 0,
+        exec => sub {
+            my( $session, $a ) = @_;
+            my $eval =  $a->evaluate($session);
+            return 1 unless $eval;
+            return 0 if ( $session->{cgiQuery}->param( $eval ));
+            return 0 if ( $session->{prefs}->getPreferencesValue( $eval ));
+            return 0 if ( $session->{SESSION_TAGS}{$eval} );
+            return 1;
+        },
+    },
     @cmpOps,
    );
 
Index: data/TWiki/IfStatements.txt
===================================================================
--- data/TWiki/IfStatements.txt	(revision 13708)
+++ data/TWiki/IfStatements.txt	(working copy)
@@ -32,6 +32,7 @@
 | =~= | regular expression match (use perl regular expressions) |
 | context | True if the current context is set (see below) |
 | defined | True if a preference variable or url parameter of this name is defined. |
+| empty | True if a preference variable or url parameter of this name has an empty value. |
 | $ | expands a URL parameter or [[TWikiVariables][TWikiVariable]]. Plugin handlers *are not called*. Built-in variables and user-defined preferences are supported. You can pass a limited subset of parameters to TWiki variables by enclosing the variable name in single quotes; for example, =$ 'VARIABLE{value}'=. The ='VARIABLE{value}'= string may *not* contain quotes (' or "). |
 examples:
 <verbatim>
Index: test/unit/Fn_IF.pm
===================================================================
--- test/unit/Fn_IF.pm	(revision 13708)
+++ test/unit/Fn_IF.pm	(working copy)
@@ -19,6 +19,7 @@
     $this->{twiki}->enterContext('test');
     $TWiki::cfg{Fnargle} = 'Fleeble';
     $TWiki::cfg{A}{B} = 'C';
+    $TWiki::cfg{A}{EMPTY} = '';
     my $u = $this->{twiki}->{users};
     my @tests = (
         { test => 'A=B', then=>0, else=>1 },
@@ -28,6 +29,7 @@
         { test => 'context test', then=>1, else=>0 },
         { test => '{Fnargle}=Fleeble', then=>1, else=>0 },
         { test => '{A}{B}=C', then=>1, else=>0 },
+        { test => 'isempty {A}{EMPTY}', then=>1, else=>0 },
         { test => '$ WIKINAME = '.$u->getWikiName($this->{twiki}->{user}),
           then=>1, else=>0 },
         { test => 'defined EDITBOXHEIGHT', then=>1, else=>0 },