The cross term is not small, it is zero

Kelvin's theorem rests on one integral vanishing: the potential flow and any solenoidal field that does not cross the boundary are orthogonal. Six perturbations of different angular order, each normalised against the geometric mean of the two energies, come out between three parts in 10¹⁷ and seven in 10¹⁵ — which is round-off, and round-off is what an identity looks like when it is integrated numerically.

3 essays call ideal-exact. The drawing above is what it returns with no arguments at all; every call below passes it something, because a placement that passes nothing draws whichever member of the family the generator happens to default to rather than the one its essay argues about.

It is also the blast radius. Changing ideal-exact changes every figure listed here at once, and on this site it may change what they assert as well as what they show — which is what has to be rebuilt and looked at before the change is believed.