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function: write_point -- save 3D location
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keyboard equiv Linux: <Alt-f>
keyboard equiv Mac: <Cmd-f>

The "SEND PNT" button writes the coordinates of
the last selected 3D location (left-click) to the
file $SUBJECTS_DIR/<subj>/tmp/edit.dat for
tkmedit, tkregister, or tksurfer.

Those other programs can read this file to go to
the same point.
