Added spec for not() in a predicate

spec/oga/xpath/compiler/calls/not_spec.rb

@@ -4,22 +4,32 @@ describe Oga::XPath::Compiler do
   describe 'not() function' do
     before do
       @document = parse('<root>foo</root>')
+
+      @root = @document.children[0]
     end
 
-    it 'returns false when the argument is a non-zero integer' do
-      evaluate_xpath(@document, 'not(10)').should == false
+    describe 'at the top-level' do
+      it 'returns false when the argument is a non-zero integer' do
+        evaluate_xpath(@document, 'not(10)').should == false
+      end
+
+      it 'returns true when the argument is a zero integer' do
+        evaluate_xpath(@document, 'not(0)').should == true
+      end
+
+      it 'returns false when the argument is a non-empty node set' do
+        evaluate_xpath(@document, 'not(root)').should == false
+      end
+
+      it 'returns itrue when the argument is an empty node set' do
+        evaluate_xpath(@document, 'not(foo)').should == true
+      end
     end
 
-    it 'returns true when the argument is a zero integer' do
-      evaluate_xpath(@document, 'not(0)').should == true
-    end
-
-    it 'returns false when the argument is a non-empty node set' do
-      evaluate_xpath(@document, 'not(root)').should == false
-    end
-
-    it 'returns itrue when the argument is an empty node set' do
-      evaluate_xpath(@document, 'not(foo)').should == true
+    describe 'in a predicate' do
+      it 'returns a NodeSet containing all matching nodes ' do
+        evaluate_xpath(@document, 'root[not(node())]').should == node_set(@root)
+      end
     end
   end
 end