Rewrite default host substitutions

Rewrite default host substitutions, ,
Any occurrences of  are replaced with the current default host, as selected by the   and related  channel option settings on the current source channel. Any occurrences of      are replaced with the value of the   th element, counting from zero, of the current default host;  thus if the current default host has been determined to be ,   will  substitute " ",   will substitute " ",  etc. If no default host is set, e.g. through the use of, then   or      rewrite rules will fail (will not be considered to have  matched).

See also:
 * defaulthost Option
 * Rewrite rule template substitutions and control sequences