Alist mapping fringe mode names to fringe widths.
Each list element has the form (NAME . WIDTH), where NAME is a
mnemonic fringe mode name (a symbol) and WIDTH is one of the
- nil, which means the default width (8 pixels).
- a cons cell (LEFT . RIGHT), where LEFT and RIGHT are
respectively the left and right fringe widths in pixels, or
nil (meaning to disable that fringe).
- a single integer, which specifies the pixel widths of both fringes.