introduce statement class for mom

lib/mom/if_statement.rb

@@ -1,5 +1,5 @@
 module Mom
-  class IfStatement < Instruction
+  class IfStatement < Statement
     attr_reader :condition , :if_true , :if_false
 
     attr_accessor :hoisted

lib/mom/instruction.rb

@@ -4,10 +4,8 @@ module Mom
   class Instruction
     attr :next_instruction
 
-    # flattening will change the structure from a tree to a linked list (and use
-    # next_instruction to do so)
+    # implement flatten as noop to avoid condition
     def flatten
-      raise "not implemented"
     end
   end
 
@@ -19,6 +17,13 @@ module Mom
     end
   end
 
+  class Statement
+    # flattening will change the structure from a tree to a linked list (and use
+    # next_instruction to do so)
+    def flatten
+      raise "not implemented"
+    end
+  end
 
 end
 

lib/mom/while_statement.rb

@@ -1,6 +1,6 @@
 
 module Mom
-  class WhileStatement < Instruction
+  class WhileStatement < Statement
     attr_reader :condition , :statements
 
     attr_accessor :hoisted