Growing and shrinking

With a length on the right, + and - grow and shrink a shape instead of combining shapes:

shape core + 2um as wider      # dilation: every edge moves 2um outwards
shape core - 200nm as narrower # erosion: every edge moves 200nm inwards

Growing rounds nothing off by itself: corners stay sharp (they are mitered). After the expression, a join chooses the corners: "miter" (the default), "round" for circular arcs, or "bevel" to cut them straight:

shape core + 2um, "round" as halo

Derived shapes replace their operands only on their own layer. A shape on another layer can grow from a waveguide without removing it, which is how claddings and exclusion zones are drawn:

layer slab {
    shape core + 2um as cladding   # core stays on si
}