In the paragraph

**
A sign-rule of #t indicates to output a plus sign (+) for positive
integers. However, if sign [!] is a pair of two strings, it means to
wrap negative numbers with the two strings. For example, ("(" . ")")
prints negative numbers in parentheses, financial style: -1.99 =>
(1.99).
**

"sign" (marked with an exclamation mark) should be replaced by "sign-rule".