ANSI Common Lisp   22 Printer   22.3 Formatted Output   22.3.5 FORMAT Pretty Printer Operations

22.3.5.1 Tilde Underscore: Conditional Newline

Without any modifiers, ~_ is the same as (pprint-newline :linear). ~@_ is the same as (pprint-newline :miser). ~:_ is the same as (pprint-newline :fill). ~:@_ is the same as (pprint-newline :mandatory).