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The overlay arrow is useful for directing the user’s attention to a particular line in a buffer. For example, in the modes used for interface to debuggers, the overlay arrow indicates the line of code about to be executed.
This variable holds the string to display to call attention to a
particular line, or nil if the arrow feature is not in use.
Despite its name, the value of this variable can be either a string
or a glyph (see Glyphs).
This variable holds a marker that indicates where to display the overlay arrow. It should point at the beginning of a line. The arrow text appears at the beginning of that line, overlaying any text that would otherwise appear. Since the arrow is usually short, and the line usually begins with indentation, normally nothing significant is overwritten.
The overlay string is displayed only in the buffer that this marker points into. Thus, only one buffer can have an overlay arrow at any given time.
You can do the same job by creating an extent with a
begin-glyph property. See Extent Properties.