ANSI Common Lisp 2 Syntax 2.1 Character Syntax 2.1.4 Character Syntax Types
220.127.116.11 Multiple Escape Characters
A pair of multiple escape characters
is used to indicate that an enclosed sequence of characters,
including possible macro characters and whitespace2 characters,
are to be treated as alphabetic2 characters
with case preserved.
Any single escape and multiple escape characters
that are to appear in the sequence must be preceded by a single escape
Vertical-bar is a multiple escape character
in standard syntax.
18.104.22.168.1 Examples of Multiple Escape Characters