Here's some additional information about making your game character move in the x direction as well as the y direction.
There's a supplemental lesson in the Bootstrap 1 materials that uses a new concept called "structs" and a new function you can use called "make-posn." Read the supplemental lesson here.
If you would like to see this in action and take a peek at the Racket code, here's a link to Blank Game with just these updates. Look for the definition statement for "update-player."
Have fun with your game, and the rest of your summer!
