Support for the XPath "mod" operator.

lib/oga/xpath/evaluator.rb

@@ -658,6 +658,22 @@ module Oga
         return on_call_number(context, left) / on_call_number(context, right)
       end
 
+      ##
+      # Processes the `mod` operator.
+      #
+      # This operator converts the left and right expressions to numbers and
+      # returns the modulo of the two numbers.
+      #
+      # @param [Oga::XPath::Node] ast_node
+      # @param [Oga::XML::NodeSet] context
+      # @return [Float]
+      #
+      def on_mod(ast_node, context)
+        left, right = *ast_node
+
+        return on_call_number(context, left) % on_call_number(context, right)
+      end
+
       ##
       # Delegates function calls to specific handlers.
       #