[ '[ @ not ] find drop ] keepd swap
[ dup length ] unless* tail-slice ; inline
+: count-head ( seq quot -- n )
+ [ not ] compose find drop ; inline
+
+: count-tail ( seq quot -- n )
+ [ not ] compose [ find-last drop ] 2keep drop
+ length swap [ - 1 - ] when* ; inline
+
:: interleaved-as ( seq glue exemplar -- newseq )
seq length dup 1 - + 0 max exemplar new-sequence :> newseq
seq [ 2 * newseq set-nth-unsafe ] each-index