simple set optimisation

the price of having simple code (generation) is that
it is sometimes stupid code that gets generated
Filter some of the really daft stuff out

lib/virtual/passes/set_optimisation.rb

@@ -0,0 +1,19 @@
+module Virtual
+
+  class SetOptimisation
+    def run block
+      block.codes.dup.each_with_index do |code , i|
+        next unless code.is_a? Virtual::Set
+        next_code = block.codes[i+1]
+        next unless next_code.is_a? Virtual::Set
+        next unless code.to == next_code.from
+        # TODO: a correct implementation would have to check that the code.to
+        #       is not used in further blocks, before being assigned to
+        new_code = Virtual::Set.new(code.from , next_code.to )
+        block.replace(code , [new_code] )
+        block.replace(next_code , [] )
+      end
+    end
+  end
+  Virtual.machine.add_pass "Virtual::SetOptimisation"
+end