J. Cabot, A. Olivé, and E. Teniente. International Conference on the Unified Modeling Language (UML), volume 2863 of Lecture Notes in Computer Science, page 44-59. San Francisco, Springer, (October 2003)
D. Akehurst, and B. Bordbar. International Conference on the Unified Modeling Language (UML), 2185/2001, page 91-103. Toronto, (October 2001)Dated; refers to OCL 1. Most suggestions have been implemented in OCL 2..
L. Mandel, and M. Victoria. World Congress on Formal Methods, 1708/1999, page 713. Toulouse, France, (September 1999)Dated as it refers to OCL 1, which lacked tuples.