(Named after the British mathematician Bertrand RUSsell (1872-1970)). Wm. Leler. Rule-based specification language based on augmented term rewriting. USed to implement constraint languages. The USer mUSt explicitly specify the tree-search and the constraint propagation. . ["Constraint Programming Languages - Their Specification and Generation", W. Leler, A-W 1988, ISBN 0-201-06243-7].