ANSI Common Lisp 2 Syntax 2.4 Standard Macro Characters 2.4.8 Sharpsign
188.8.131.52 Sharpsign Asterisk
The notations #* and #0* each denote an empty bit vector.
Regardless of whether the optional numeric argument n is provided, the token that follows the asterisk is delimited by a normal token delimiter. However, (unless the value of *read-suppress* is true) an error of type reader-error is signaled if that token is not composed entirely of 0's and 1's, or if n was supplied and the token is composed of more than n bits, or if n is greater than one, but no bits were specified. Neither a single escape nor a multiple escape is permitted in this token.