pub fn assemble_sign(lambda_det_sign: Sign, den_signs: &[Sign]) -> SignExpand description
Combine the sign of the homogenised determinant Λ′ with the
per-configuration denominator flip.
When an implicit point (λ/d) enters a determinant row, clearing the
denominator multiplies the determinant by d (degree = the denominator’s
multiplicity in that configuration). The geometric sign therefore equals
sign(Λ′) flipped once per NEGATIVE odd-multiplicity denominator.
den_signs lists ONLY the odd-multiplicity denominator signs — squared
denominators (e.g. the TPI III orient3d case, D′=(d1d2d3d4)²) cannot
change the sign and MUST NOT be included. Getting this wrong silently
inverts inside/outside for ~half of real cuts (the per-config rule, not a
blanket XOR over all negatives — see the spec’s REFUTATION-FIX).
A Zero denominator means a degenerate / at-infinity construction (e.g. the
LPI line is parallel to the plane, d=0): the predicate is undefined, so we
return Zero. Valid implicit points (built only for genuinely-crossing
edges) never have a zero denominator.