Ifacedefined (number)

   Synopsis:
      Tests whether the specified substring actually matched the line, or not
      
   Notes:
      The interface notification list contains all the notifications received,
         and not yet processed, as the result of an earlier SETTRIG statement.
      This function operates on the current notification (usually the first one
         see the help for SKIPIFACE).

      You might want the trigger to match any of the following lines:

         The warrior hits you with axe
         The warrior hits you with handaxe
         The warrior hits you with poleaxe

      ...but not this line:

         The warrior hits you with surtaxes

      In that case, you might use the following regular expression:
      
         The warrior hits you with (hand|pole)?axe'

      Additionally, you might want to capture the type of weapon used, and
         for that, you would add substrings:

            The warrior hits you with ((hand)|(pole))?axe'

      When the regular expression matches a line containing "handaxe" or
         "poleaxe", Perl produces a list of substrings containing two items, one
         of them an undefined value:
         
            ( "hand", undefined )
            ( undefined, "pole" )

      The Ifaceselect () function returns an empty string in place of the
         undefined value, but it might also return an empty string if the line
         contained just "axe" and not "handaxe" or "poleaxe".
      You can use this function if you specifically want to test for undefined
         values. The specified substring is tested. The first substring is
         numbered 1, the second is numbered 2, and so on.
      This function returns 1 if the specified substring exists, and is defined.
         If the specified substring exists but is not defined, returns -1. If
         the specified substring doesn't exist, returns 0.

   Compatibility:
      Since Axbasic v1.003
