Common Lisp the Language 2nd Edition

2.5.3. Bit-Vectors

A bit-vector can be written as the sequence of bits contained in the string preceded by #*; any delimiter character such as whitespace will terminate the bit-vector syntax. For example:

#*10110     ;A five-bit bit-vector; bit 0 is a 1
#*          ;An empty bit-vector

The bits notated following the #* taken from left to right occupy locations within the bit-vector with increasing indices. The leftmost notated bit is bit-vector element number 0 the next one is element number 1 and so on.

The function prin1 will print any bit-vector (not just a simple one) using this syntax but the function read will always construct a simple bit-vector when it reads this syntax.