ui/src/split/Filter.i3

 Copyright (C) 1992, Digital Equipment Corporation                         
 All rights reserved.                                                      
 See the file COPYRIGHT for a full description.                            
                                                                           
 Last modified on Mon Feb 24 13:53:11 PST 1992 by muller                   

<*PRAGMA LL*>

 A Filter.T is a Split.T with at most one child. 


INTERFACE Filter;

IMPORT Split, VBT;

TYPE
  T <: Public;
  Public = Split.T OBJECT METHODS
     <* LL.sup <= VBT.mu *>
     init(ch: VBT.T): T
   END;

 The call v.init(ch) initializes v as a filter with
   child ch and returns v. 


 Split.Move on a filter is a noop.  Split.Insert replaces the child,
   if any, and detaches it. 


PROCEDURE Child(v: T): VBT.T;
<* LL.sup = VBT.mu *>

 Return the child of v, or NIL if there is no child. 
 Filter.Child(v) is equivalent to Split.Succ(v, NIL). 


PROCEDURE Replace(v: T; ch: VBT.T): VBT.T;
<* LL.sup = VBT.mu *>

 Replace v's child by ch, detach and return v's old child, and
   mark v for redisplay.  
 Filter.Replace is similar to Split.Replace, except that it
   returns the old child instead of taking the old child as an argument,
   and if ch is NIL it is similar to Split.Delete.  


END Filter.