-------------------------------------------
functs,vars (left-click): to read computed fieldsign
  set fs <$hemi.fs>
  read_fieldsign
  set fm <$hemi.fm>
  read_fsmask
-------------------------------------------

The "R" button on the "fs:" entry line reads the
fieldsign from the file whose name is the current
value of $fs (entry at left).

The tcl interface then also reads the fieldsign
mask (statistical mask) from the file whose name
is the current value of $fs with the .fs suffix
replaced by an .fm suffix.
