⌺ — Stencilf⌺S Y applies f to neighbourhoods of sizes S on leading axes. Windows receive prototype fill at edges.
{+/,⍵}⌺3⊢1 2 3 4 ⍝ 3 6 9 7
⍵ is the window; ⍺ gives signed padding counts per axis: positive at the start, negative at the end. A two-row specification gives sizes then movements.
{+/,⍵}⌺(2 1⍴3 2)⍳8 ⍝ 3 9 15 21
Results assemble with fill. DOMAIN: nonpositive sizes/movements, window too large for an axis.