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tickbox variable: $flag2d -- shrink() -> shrink2d()
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default: 0 (FALSE)

The "2d" tickbox (F3 panel, lower middle) toggles
whether or not to use an alternate C/tcl 'shrink'
function, shrink2d().

The shrink2d() function is designed for
flattening cut surfaces, so it includes an
additional force that is normal to the boundary
of a cut edge.

The strength of this force is controlled by $wbn
("weight of the boundary normal").  This force
prevents inward compression and curling of the
unbalanced edge vertices.
