'INVALID
One or both elements are invalid. Data return format: (INVALID
nil). For distance between a pad and non-pad element, use the
element subclass, if the non pad element is on etch. If not, use
the t_layer which defaults to the active layer, if undefined. If
non-etch and the pad are undefined on the subclass, determine
the subclasses on which the pad is defined and return those
layers (XXX). For distance between two pads, return gap based
upon the active etch subclass, if t_layer is nil. Otherwise use
t_layer to determine gap. If one or both pads do not exist on
that layer, there is an error.