ANSI Common Lisp 2 Syntax
2.4 Standard Macro Characters
If the reader encounters a macro character,
then its associated reader macro function
is invoked and may produce an object to be returned.
This function may read the characters
following the macro character in the stream
in any syntax and return the object represented by that syntax.
Any character can be made to be a macro character.
The macro characters defined initially in a conforming implementation
2.4.9 Re-Reading Abbreviated Expressions