15.58.6  FILL 

Fills  the  symbols  to  make them "more solid".  You will have to use the
command SET FILL to achieve the desired effect.  Many symbols can  not  be
filled properly, so you must just try until you find a satisfactory one.

Of  the  normal  ASCII  character  set  the "0O8SZ$" look pretty good when
filled.  The centered symbol 1o, 2o, 6o, 8o look good when filled.  3o and
4o look bad when filled, while the rest to not fill at all.