Layers in the 3xx series use all the available syntax sugar for SubX programs. They're used by the Mu translator (apps/mu.subx) and can also be called from Mu programs if they meet certain criteria: - There's a signature for them in 400.mu - Inouts on the stack, outputs in registers - Valid Mu types everywhere (Mu's type system isn't expressive enough for everything SubX does in rare situations.) - No way to for an `addr` to escape a function. No `(... addr ... addr ...)` inouts, and no `(... addr ...)` outputs. While functions _can_ be called, not all SubX functions meeting these criteria _should_ be called. In particular, avoid exporting functions that could be misused. A classic example is trying to add a `size-of` operator. If you're doing that you're likely going to rely on programmers to use it correctly. Mu tries to be idiot-proof. Even if SubX requires greater care, using SubX primitives from Mu should not.