ANSI Common Lisp   22 Printer   22.3 Formatted Output   22.3.8 FORMAT Miscellaneous Operations

22.3.8.3 Tilde P: Plural

If arg is not eql to the integer 1, a lowercase s is printed; if arg is eql to 1, nothing is printed. If arg is a floating-point 1.0, the s is printed.

~:P does the same thing, after doing a ~:* to back up one argument; that is, it prints a lowercase s if the previous argument was not 1.

~@P prints y if the argument is 1, or ies if it is not. ~:@P does the same thing, but backs up first.

 (format nil "~D tr~:@P/~D win~:P" 7 1)  "7 tries/1 win"
 (format nil "~D tr~:@P/~D win~:P" 1 0)  "1 try/0 wins"
 (format nil "~D tr~:@P/~D win~:P" 1 3)  "1 try/3 wins"