ANSI Common Lisp   2 Syntax   2.4 Standard Macro Characters   2.4.8 Sharpsign

2.4.8.7 Sharpsign B

#Brational reads rational in binary (radix 2). For example,

 #B1101 ==13 ;11012
 #b101/11 ==5/3

The consequences are undefined if the token immediately following the #B does not have the syntax of a binary (i.e., radix 2) rational.