Reinder J. Bril Loe M. G. Feijs André Glas René L. Krikhaar Thijs Winter Hiding Expressed Using Relation Algebra with Multi-Relations - Oblique Lifting and Lowering for Unbalanced Systems. 33-44 2000 CSMR http://www.computer.org/proceedings/csmr/0546/05460033abs.htm db/conf/csmr/csmr2000.html#BrilFGKW00