Examples

   > IEXC PIX                interactively exclude individual pixels
   > IEXC SOURCE LIST=PSLIST RAD=5      exclude pixels up to a radius
                                        of 5 axis units from positions
                                        in given source list
   > IEXC CIRC OUTSIDE       exclude pixels outside given circle
   > IEXC REG CURR           exclude pixels in current region as
                             previously defined by IREGION