added link to visitor pattern

from_oop_to_fp_-_inheritance_and_the_expression_problem.md

@@ -235,7 +235,7 @@ In case `Base.hs` would get too long in our Haskell version
 we could split in into `Step.hs` and `Display.hs`.
 But still, if we would like to add a new object, we would have to edit
 *all* the functions. There would not be one single point to do it.
-Vice versa, the OOP solution would make it difficult to add a new
+Vice versa, the OOP solution (without [visitor pattern](http://en.wikipedia.org/wiki/Visitor_pattern)) would make it difficult to add a new
 function. We would have to edit the files of all derived classes.
 
 Perhaps this is not a big surprise. In OOP our code is structured by objects,