Mapping from logical to physical fringe indicator bitmaps.
The value is an alist where each element (INDICATOR . BITMAPS)
specifies the fringe bitmaps used to display a specific logical
fringe indicator.
INDICATOR specifies the logical indicator type which is one of the
following symbols: `truncation' , `continuation', `overlay-arrow',
`top', `bottom', `top-bottom', `up', `down', empty-line', or `unknown'.
BITMAPS is a list of symbols (LEFT RIGHT [LEFT1 RIGHT1]) which specifies
the actual bitmap shown in the left or right fringe for the logical
indicator. LEFT and RIGHT are the bitmaps shown in the left and/or
right fringe for the specific indicator. The LEFT1 or RIGHT1 bitmaps
are used only for the `bottom' and `top-bottom' indicators when the
last (only) line has no final newline. BITMAPS may also be a single
symbol which is used in both left and right fringes.