ACL2 accepts 256 distinct characters, which are the characters
 obtained by applying the function code-char 
 to each
 integer from 0 to 255.  Among these, Common Lisp designates certain
 ones as *standard-characters*, namely those of the form
 (code-char n) where n is from 33 to 126, together with
 #\Newline and #\Space.  The actual standard characters may
 be viewed by evaluating the constant expression
 *standard-chars*.
The standard character constants are written by writing a hash mark followed by a backslash (#\) followed by the character.
 The function characterp 
 recognizes characters.  For more
 details, See characters 
.