minor

test/fragments/helper.rb

@@ -9,6 +9,7 @@ module Fragments
   def check
     expressions = Virtual.machine.boot.compile_main @string_input
     @expect.each_with_index do | should , i |
+      assert expressions[i].is_a?(Virtual::Slot) , "compiles should return slots, not #{should.class}"
       assert_equal should , expressions[i].class
     end
 #    Virtual.machine.run_passes