 
   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"
 

      expression.cloneAndAppend(Division.build())
  class_mixer(DivisionCommand)

  class PiCommand
    invoke: (expression)->
      expression.cloneAndAppend(Pi.build())
  class_mixer(PiCommand)

  class SquareRootCommand
    invoke: (expression)->

      value = expression.evaluate().value() || 0
      root = Math.sqrt(parseFloat(value))
      unless isNaN(root)


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

