ANSI Common Lisp   2 Syntax   2.4 Standard Macro Characters   2.4.8 Sharpsign

2.4.8.15 Sharpsign Equal-Sign

#n=

#n=object reads as whatever object has object as its printed representation. However, that object is labeled by n, a required unsigned decimal integer, for possible reference by the syntax #n#. The scope of the label is the expression being read by the outermost call to read; within this expression, the same label may not appear twice.