heterodyne: Protect another FINDBACK and distinguish the two contexts.

There can be some receptors with mostly blanked out values bue
sufficient good values are the spectral ends to pass through the
high-frequency-interference filter, but have no good values for the
low-frequency test.

Distinguish between the two cases in the same primitive through
different commentary.
heterodyne: Protect FINDCLUMPS against zero or negative contours.

The data can be all zero.