GN_Fpb, push back a character into filter

RST 20H, DEFW $2C09

IN:

     IX = filter handle

     Fc = 0

     Fc = 1
     A = error code:
          RC_HAND ($08), bad filter handle
          RC_PUSH ($0E), push back error
               1. never read from filter
               2. Already pushed back one character
          RC_EOF ($09), filter is empty

     ..BCDEHL/IXIY same
     AF....../.... different

At most one character can be pushed back without an intervening read, otherwise a push back error is given.

Related calls:

GN_Flc, close filter
GN_Flf, flush filter
GN_Flo, open filter
GN_Flr, read from filter
GN_Flw, write character to filter