behind

rpic extension, inspired by Pikchr. behind paints the current object under an already-drawn one — highlights, backgrounds, halos — without reordering anything semantically.

.PS
A: box "content" fit
box wid A.wid + 0.2 ht A.ht + 0.2 at A shaded "honeydew" outlined "darkgreen" behind A
.PE

Rules

• Only backend paint order changes. Evaluation order, labels, anchors, ordinals (last box, 2nd box), SVG ids (s0, s1) and animation targets all keep source order.
• Layers cascade: an object drawn behind something that is itself behind another lands below both.
• Without behind, rpic keeps the classic natural order — later objects paint over earlier ones:

.PS
box "under" fit shaded "mistyrose"
box "over" fit shaded "honeydew" at last box + (0.35, -0.15)
.PE

Design notes: docs/pikchr.md.