Antonio L. Furtado Clesio Saraiva dos Santos José Mauro Volkmer de Castilho Dynamic modelling of a simple existence constraint. 73-80 1981 6 Inf. Syst. 1 db/journals/is/is6.html#FurtadoSC81 http://dx.doi.org/10.1016/0306-4379(81)90019-3