 
   class Pi extends ExpressionComponent
     toDisplay: -> "&pi;"
-    eval: ->
+    eval: ()->
       Number.build(value: Math.PI)
 
   class_mixer(Pi)
 
   class Addition extends ExpressionComponent
     toDisplay: -> "+"
+    isOperator: -> true
     eval: (evaluation, pass)->
       return if pass != "addition"
 


  class Subtraction extends ExpressionComponent
    toDisplay: -> "-"
    eval: (evaluation, pass)->
      return if pass != "addition"


  class_mixer(Multiplication)

  class Division extends ExpressionComponent
    isOperator: -> true
    toDisplay: -> "&divide;"
    eval: (evaluation, pass)->
      return if pass != "multiplication"
