The association of scientific frames with suitable calibration images is
achieved by selecting and ranking all calibration
frames which for a given scientific exposure match a set of user defined
selection criteria.
One may expand the search by submitting
a ``second choice'' set of criteria when too few frames match the original one.
For instance, this may in natural language be expressed as: ``Find for each
scientific frame 2 dark exposures which have been observed within the same
night. If not found, look for dark exposures observed within 3 days.''