Overlay Type
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An "overlay" specifies properties that apply to a part of a buffer.
Each overlay applies to a specified range of the buffer, and contains a
property list (a list whose elements are alternating property names and
values). Overlay properties are used to present parts of the buffer
temporarily in a different display style. Overlays have no read
syntax, and print in hash notation, giving the buffer name and range of
positions.
Note:Overlays, for how to create and use overlays.