! right of the index, plus one that's not equal, if requested.
:: adjusted-head-slice ( n elt plus-one? seq -- slice )
n seq elt x '[ x _ = not ] find-from drop seq swap
- [ plus-one? [ 1 + ] when bound head-slice ] when* ;
+ [ plus-one? [ 1 + ] when index-or-length head-slice ] when* ;
! : data-rect ( data -- rect )
! [ [ first x ] [ last x ] bi ] keep