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variable: $wa -- area force: area_shrink, shrink_2d()
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default: 0.5

The floating point parameter, $wa ("weight of
area"), (F3 panel, lower left) controls the
strength of the tangential 'force' that tries to
restore original area associated with this vertex
(sum of 1/3 of the area of each surrounding
face).

Only relevant to:

 area_shrink() -- SHRINK with "ar" ticked
 shrink_2d() -- SHRINK with "2d" ticked
