rename conditional to if expression

lib/ast/if_expression.rb

@@ -0,0 +1,15 @@
+module Ast
+  class IfExpression < Expression
+    attr_reader  :cond, :if_true, :if_false
+    def initialize cond, if_true, if_false
+      @cond, @if_true, @if_false = cond, if_true, if_false
+    end
+    def inspect
+      self.class.name + ".new(" + cond.inspect + ", "+ 
+        if_true.inspect +  ","  + if_false.inspect + " )"  
+    end
+    def attributes
+      [:cond, :if_true, :if_false]
+    end
+  end
+end