ANSI Common Lisp 22 Printer 22.3 Formatted Output 22.3.4 FORMAT Printer Operations
188.8.131.52 Tilde A: Aesthetic
An arg, any object, is printed without escape characters (as by princ). If arg is a string, its characters will be output verbatim. If arg is nil it will be printed as nil; the colon modifier (~:A) will cause an arg of nil to be printed as (), but if arg is a composite structure, such as a list or vector, any contained occurrences of nil will still be printed as nil.
~mincolA inserts spaces on the right, if necessary, to make the width at least mincol columns. The @ modifier causes the spaces to be inserted on the left rather than the right.
~mincol,colinc,minpad,padcharA is the full form of ~A, which allows control of the padding. The string is padded on the right (or on the left if the @ modifier is used) with at least minpad copies of padchar; padding characters are then inserted colinc characters at a time until the total width is at least mincol. The defaults are 0 for mincol and minpad, 1 for colinc, and the space character for padchar.