This v4.1.x fork implements Scalar52::sub with TWO loops (loop0 = borrow
chain; loop1 adds L &&& underflow_mask — the arithmetic-mask constant-time
conditional), a genuinely different code path from upstream v5's
subtle-based conditional_add_l. Verified per R2 against this fork's own gen:
- sub_loop_spec (= loop0): borrow chain, verbatim technique from dalek
- sub_loop1_zero_spec / sub_loop1_one_spec: the masked-L add, both mask
values (0 / 2^64-1), full carry chains
- sub_val_spec: denote(sub a b) = denote a - denote b in ZMod l; the
underflow mask um = ((borrow>>>63) XOR 1) - 1 resolved per case
- add_loop_spec + add_val_spec: denote(add a b) = denote a + denote b
(v4 add_loop extracts token-identical to v5; composition through this
fork's own sub_val_spec)
check-scalar.sh: full manifest, 5/5 kernel axiom audit
[propext, Classical.choice, Quot.sound], green at 300-400s/4096MB.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
