# height of the sprite
var height: Int = 100 is writable
+ # X coordinate of left side.
fun left: Int do return x - width/2
+
+ # X coordinate of right side.
fun right: Int do return x + width/2
+
+ # Y coordinate of top.
fun top: Int do return y - height/2
+
+ # Y coordinate of bottom.
fun bottom: Int do return y + height/2
# x velocity (applied by `update')