User blog comment:Simply Beautiful Art/Fast Array Iteration Leaping/@comment-5529393-20170404001617/@comment-31663462-20170404002450

You can have @'s next to each other. Then the 0's and the a go right in front of that last @ and behind all previous @'s, and then it slowly cascades from there.

F 3 # 5@4 # 1@0 # 2 = F 3 # 5@4 # 0 # 0 # 0 # 3 # 1@0 # 1

And now apply the rule for things of the form F a # ? # .... # ? # 0 # b # .... # c.... which I fixed (it used to be a string of zeroes rule, but it works for a string of ?'s followed by a zero followed by a number.