#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.