GHCi, version 7.6.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main             ( street.hs, interpreted )
Ok, modules loaded: Main.
*Main> 
   yy     
   yy     
   yyzzzzz
   yyzzzzz
xxxyyzzzzz
xxxyyzzzzz
----------
*Main> Leaving GHCi.