Merge branch 'working'

* working:
  replace tattoo image

diff --combined topics/week4_fixed_point_combinators.mdwn
index 1ed99d2,a16d7c5..ed22cff
--- 1/topics/week4_fixed_point_combinators.mdwn
--- 2/topics/week4_fixed_point_combinators.mdwn
+++ b/topics/week4_fixed_point_combinators.mdwn
@@@ -293,7 -293,6 +293,7 @@@ The strategy we will present will turn 
  finding a fixed point for any lambda term.
  
  
 +<a id=deriving-y></a>
  ## Deriving Y, a fixed point combinator ##
  
  How shall we begin?  Well, we need to find an argument to supply to
@@@ -534,7 -533,7 +534,7 @@@ to *the tail* of the list we were evalu
  
  ## Fixed-point Combinators Are a Bit Intoxicating ##
  
- [[tatto|/images/y-combinator-fixed.jpg]]
+ [[tatto|/images/y-combinator-fixed.png]]
  
  There's a tendency for people to say "Y-combinator" to refer to fixed-point combinators generally. We'll probably fall into that usage ourselves. Speaking correctly, though, the Y-combinator is only one of many fixed-point combinators.