Return equivalent geometry spec for FRAME as a cons with car `+'.
A geometry specification equivalent to SPEC for FRAME is returned,
where the value is a cons with car `+', not numeric.
SPEC is a frame geometry spec: (left . VALUE) or (top . VALUE).
If VALUE is a number, then it is converted to a cons value, perhaps
relative to the opposite frame edge from that in the original spec.
FRAME defaults to the selected frame.
Examples (measures in pixels) -
Assuming display height=1024, frame height=600:
top 300 below display top: (top . 300) => (top + 300)
(top + 300) => (top + 300)
bottom 300 above display bottom: (top - 300) => (top + 124)
(top . -300) => (top + 124)
top 300 above display top
(= bottom 724 above display bottom): (top + -300) => (top + -300)
bottom 300 below display bottom
(= top 724 below display top): (top - -300) => (top + 724)
In the 3rd, 4th, and 6th examples, the returned value is relative to
the opposite frame edge from the edge indicated in the input spec.