CustomOp

A registry for user-named operations, in macro form: a custom op is a function that expands into primitive ADouble nodes when it is recorded. Its adjoint is whatever the recorded sub-graph produces, so it works on every backend with no engine change (Seam 3, composable subset).

CustomOp.registerUnary("softplus", (rec, x) -> Smooth.ramp(rec, x, 0.05));
SDouble y = CustomOp.unary("softplus").apply(rec, x);

A fused op with a hand-written {forward(x), adjoint(x, gbar)} and per-backend code generation — for kernels that cannot be expressed in the primitive set at all — is a later engine feature. For everything expressible as a composition (which is most of what a quant registers) this form is equivalent and needs nothing from the engine.