from preamble:

"Change Set:		BinaryRelation
Date:			27 March 2003
Author:			Dr Richard A. O'Keefe

This adds a collection class I've often wanted, a many<->many binary
relation.
Basically, a BinaryRelation is like a Set of Associations, but it can
easily be
navigated in either direction.  This is a preliminary version sent out
to invite
comment.  Are the new method names well chosen?  Are there other
operations
which should be included?  I've wondered about renaming this to
ManyToManyBinaryRelation and also having OneToManyBinaryRelation and
ManyToOneBinaryRelation.  Would that be a good idea?  It's mainly the
interface
I would like criticised, although I'd like to fix any implementation
errors noticed.
There are other possible implementations."!
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