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funct: swap_valval2_valbakval2bak -- swap 2 complex
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keyboard equivalent: none

The "swapcplx" button (F3 panel, bottom right)
swaps two complex numbers at each vertex in
.val/.val2 and .valbak/.val2bak at each vertex,
which is useful during calculation of visual
field sign.

This function can also by accessed from the
standard F2 panel by using buttonbar from a
R-click on the "S/V" button on the "val:" line
(=shift-mid-click S/V).

See R-click help for "S/V" button for details.
