Copyright (C) 1992, Digital Equipment Corporation                         
 All rights reserved.                                                      
 See the file COPYRIGHT for a full description.                            
 Last modified on Thu Jun 10 13:48:22 PDT 1993 by meehan 
      modified on Mon Feb  1 00:36:23 PST 1993 by mhb    
      modified on Tue Jun 16 13:08:36 PDT 1992 by muller 
      modified on Fri Mar 27 01:47:38 1992 by steveg
The MultiFilter interface defines the functionality that is common to all clients of multi-filters; namely, retrieving and changing a multi-filter's multi-child.

A multi-filter is a multi-split with at most one child. Thus, you can use the procedures in the MultiSplit interface on a VBT that is a multi-filter. The semantics of the MultiSplit procedures on a multi-filter should be obvious, with the following exceptions: MultiSplit.Move on a multi-filter is a no-op, and MultiSplit.Insert on a multi-filter replaces the child, if any.

INTERFACE MultiFilter;


A MultiFilter.T is a VBT.T with a MultiClass.Filter in its property set.
 The following procedures can accept either a MultiFilter.T
   or a Filter.T as the first argument.  If the first argument
   is not a MultiFilter.T, the procedure just calls the corresponding
   procedure in the Filter interface. 

<* LL.sup = VBT.mu *>
Return the child of v, or NIL if there is no child.

PROCEDURE Replace (v, ch: VBT.T): VBT.T;
<* LL.sup = VBT.mu *>
Replace v's child by ch and return v's old child.
 MultiFilter.Replace is similar to MultiSplit.Replace,
   except that it returns the old multi-child instead of taking
   the old multi-child as an argument, and if ch is NIL it is
   similar to MultiSplit.Delete. 

END MultiFilter.