
Allegro CL 
ANSI Common Lisp 2 Syntax 2.4 Standard Macro Characters 2.4.8 Sharpsign
2.4.8.10 Sharpsign R#nR #radixRrational reads rational in radix radix. radix must consist of only digits that are interpreted as an integer in decimal radix; its value must be between 2 and 36 (inclusive). Only valid digits for the specified radix may be used. For example, #3r102 is another way of writing 11 (decimal), and #11R32 is another way of writing 35 (decimal). For radices larger than 10, letters of the alphabet are used in order for the digits after 9. No alternate # notation exists for the decimal radix since a decimal point suffices. The next figure contains examples of the use of #B, #O, #X, and #R.
The consequences are undefined if the token immediately following the #nR does not have the syntax of a rational in radix n. 