ANSI Common Lisp 2 Syntax 2.4 Standard Macro Characters 2.4.8 Sharpsign
220.127.116.11 Sharpsign X
#Xrational reads rational in hexadecimal (radix 16). The digits above 9 are the letters A through F (the lowercase letters a through f are also acceptable). For example,
#xF00 ==3840 #x105 ==261 ;10516
The consequences are undefined if the token immediately following the #X does not have the syntax of a hexadecimal (i.e., radix 16) rational.