Function usage:
  SUBSTITUTE(text [,
      attrs [, inline]])
Command usage:
  /SUBSTITUTE [-aattrs] [-p]
      text
When called from a trigger (directly or indirectly), the entire triggering line is replaced with text. After a /substitute, it will appear as if text is what caused the trigger; the original line is lost. In particular, this means when /substitute is called from a fall-thru trigger, triggers of lower priority will be compared against text instead of the original line.
Options and arguments:
-aattrs
  attrs
        -p
  "on" or 1
        @{attr} strings as commands to
        set attributes inline, as
        in /echo.  (See
        /echo).
  Example:
On a mud that uses MUFpage, you could set your #prepend string to "##page>", and define a trigger like:
/def -ah -t"##page> *" hilite_mufpage = /substitute %-1
This will match no matter what page format the sender uses, and strip off the "##page>" so you never see it.
For another example, see /replace.
See: triggers