boundlab.diff.zono3.tighten_diff

boundlab.diff.zono3.tighten_diff(sub, diff)

Pick, per element, the narrower of two sound enclosures of the difference.

sub is x_out − y_out — exact whenever the branch relaxations share their error symbols with the difference — and diff is the lineariser’s dedicated form. Neither dominates the other: sub wins when the branches cancel, diff wins when the lineariser exploits the strip constraint on x − y. Both enclose the true difference, so any per-element choice is sound.