Brigitte Servatius Walter Whiteley Constraining Plane Configurations in Computer-Aided Design: Combinatorics of Directions and Lengths. 136-153 1999 12 SIAM J. Discrete Math. 1 http://epubs.siam.org/sam-bin/dbq/article/30734 db/journals/siamdm/siamdm12.html#ServatiusW99