Added Ruby::Node#not

This is a shortcut for "!foo". Using this method one doesn't have to
worry about how the "!" operator binds. For example, this:

    !foo.or(bar)

would be parsed/evaluated as this:

    !(foo.or(bar))

when instead we want it to be this:

    (!foo).or(bar)

Using explicit parenthesis leads to ugly code, so now we can do this
instead:

    foo.not.or(bar)

lib/oga/ruby/node.rb

@@ -84,6 +84,17 @@ module Oga
         Node.new(:or, [self, other])
       end
 
+      ##
+      # Returns a node that evaluates to its inverse.
+      #
+      # For example, a variable `foo` would be turned into `!foo`.
+      #
+      # @return [Oga::Ruby::Node]
+      #
+      def not
+        !self
+      end
+
       ##
       # Returns a node for Ruby's "is_a?" method.
       #

lib/oga/xpath/compiler.rb

@@ -302,7 +302,7 @@ module Oga
               .followed_by(throw_message(:skip_children))
           end
 
-          next_check = (!check).or(parent != doc_node.parent).if_true do
+          next_check = check.not.or(parent != doc_node.parent).if_true do
             send_message(:next)
           end
 
@@ -343,7 +343,7 @@ module Oga
               .followed_by(throw_message(:skip_children))
           end
 
-          next_check = (!check).if_true { send_message(:next) }
+          next_check = check.not.if_true { send_message(:next) }
 
           match_node = backup_variable(node, doc_node) do
             process(ast, node, &block).if_true do

spec/oga/ruby/node_spec.rb

@@ -61,6 +61,16 @@ describe Oga::Ruby::Node do
     end
   end
 
+  describe '#not' do
+    it 'returns a send Node' do
+      node     = described_class.new(:lit, %w{foo})
+      inverted = node.not
+
+      inverted.type.should == :send
+      inverted.to_a.should == [node, '!']
+    end
+  end
+
   describe '#is_a?' do
     it 'returns a is_a? call Node' do
       left = described_class.new(:lit, %w{number})