Colmerauer, Alain (1981). Personal communication.
Kempe, A.B. (1879). On the geometrical problem of four colors. Amer. J. Math. 2, 193-204.
Gallaire, Hervé (1981). Personal communication.
Kowalski, Robert (1979). Logic for Problem Solving, New York: North-Holland.
Pereira, Luis Moniz and Antonio Porto (1980). Selective Backtracking for Logic Programs, Departamento de Informatica, Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa, Lisboa, Portugal.