  Solving a system of nonlinear inequalities is an important problem for which
conventional numerical analysis has no satisfactory method. With a
box-consistency algorithm one can compute a cover for the solution set to
arbitrarily close approximation. Because of difficulties in the use of
propagation for complex arithmetic expressions, box consistency is computed
with interval arithmetic. In this paper we present theorems that support a
simple modification of propagation that allows complex arithmetic expressions
to be handled efficiently. The version of box consistency that is obtained in
this way is stronger than when interval arithmetic is used.
