cell f(x) fallback %0 { … } — a rewrite rule; %0 means "on failure, pass argument 0 through". fallback other names another cell.
SPLIT CAT LSLICE HSLICE REPEAT · slices x[1:] x[:-1] x[0] x[::-1] · ZERO ONE
wire fb ~ x — a bus as wide as x whose driver comes later: the one forward reference, and the only way to close a loop.
Widths are never declared. Recursion ends when a bus stops dividing.