BFM-decomp/cookbook/C0130.md

§119 — Two levers on the SAME axis, opposite directions: test the off-diagonal (Phase 29 T89)

family_sweep carries two declaration-axis levers that pull opposite ways:

flag	bends	correct when
--fix-def-sig	the definition -> the shared-header decl	the header is right and the draft's def is wrong
--normalize-self-decls	the in-TU declarations -> the definition	the definition is byte-true and a decl disagrees

0x80161c98 needed exactly one of each: its byte-true def is (int, u32) (target emits sltiu), while engine_core.h says (s32, s32) and a TU decl disagreed. So:

• both flags (T79): --fix-def-sig rewrote the def to s32 -> slti -> byte DIFF. 0/138.
• neither flag (T84/T88): correct sltiu codegen, but conflicting types for func_80161D20 from the in-TU decl -> will not compile. 0/138.
• --normalize-self-decls only: def stays byte-true, the divergent decl is dropped and its calls cast. 138/138.

Three sweeps across three sessions read as a compiler wall because all three tested the diagonal of the 2x2 (both on, both off) and never the off-diagonal.

The law: when two levers act on the same axis in opposite directions, "tried it with the flags" and "tried it without" cover half the matrix. Enumerate the off-diagonal before calling the family blocked — especially when one lever's own docstring says it rewrites what the other preserves.

Measured: 0x80161c98 0 -> 138/138 (130 distinct); +23 across the remaining still-zero families. Corollary to §118's caution: a per-family blocker can be a flag combination, not a defect.