Things wrong:
- indentation
- the dc callback didn't reset the dc state
(this is okay if you only use the pict
in the interactions window in drracket,
because drracket protects itself, but it
is not okay if you use them in slideshow
or something)
- the dc callback didn't use the 'dx' and 'dy'
arguments properly
(you could see this going wrong if you put
two of the picts together in an hc-append
and looked at the compound pict in the
interactions window)
