def process_proc(proc_obj, context=nil, filename=nil)
      if context.is_a?(Binding)
        filename ||= '(erubis)'
        return eval(proc_obj, context, filename)
      else
        context = Context.new(context) if context.is_a?(Hash)
        return context.instance_eval(&proc_obj)
      end
    end
def process_proc(proc_obj, context=nil, filename=nil)
      if context.is_a?(Binding)
        filename ||= '(erubis)'
        return eval(proc_obj, context, filename)
      else
        context = Context.new(context) if context.is_a?(Hash)
        return context.instance_eval(&proc_obj)
      end
    end
helper method evaluate Proc object with contect object.
 context may be Binding, Hash, or Object.