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funct (left-click): write curvature
  set curv <outfile>
  write_binary_curv
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The "W" button on the "curv:" entry line
writes out current local surface curvature as
file: $curv (name in entry to left). Must confirm
overwrite, or use write_binary_curv (doesn't ask).

To make the sulc file, set sulcflag to 1, unfold
surface with SHRINK, set the output file name in
$curv, and write_binary_curv.
