[racket] Rosetta: Pascal's triangle/Puzzle

 Hi!

I just added the solution to the Pascal's triangle/Puzzle in Rosetta:
http://rosettacode.org/wiki/Pascal%27s_triangle/Puzzle#Racket

Any comments and improvements are welcome.

Some comments:

The solution is based in the Clojure solution, but it's not a direct
translation, so I didn't mark it with {{trans|clojure}}

I used the same structure to represent the content of the cells and
the equations. So the sign of the independent values in the equations
are "wrong". Any suggestion?

The structure of cell-add and cell-sub is very similar, but I
considered that refactoring it as a cell-map makes the solution less
intelligible.

Gustavo