1. Apparatus for calculating a first result modal interval dependent on a first modal interval, each said modal interval defined by first and second endpoints each comprising sign, fraction and exponent bit fields, said apparatus comprising: a) a first operand register for holding the first and second endpoints, and for providing a first operand signal specifying the contents of the first modal interval endpoints' fields; b) a first analysis element receiving the first operand signal, creating first and second truth tables dependent on the first operand signal, and encoding the first and second truth tables in a first truth tables signal; c) a first logic array receiving the first operand signal and the first truth tables signal, and providing a first “empty” bit value signal dependent on the sign, fraction, and exponent field contents of the first and second endpoints; and d) a first computational element receiving the first operand signal and the “empty” bit value signal, and performing a computation associated with the first computational element using the data encoded in the first operand signal and the “empty” bit value signal, and providing a result signal encoding the first result modal interval as a result of the computation by the first computational element.