1. Programmable logic array apparatus comprising: a plurality of logic modules, each of which has a plurality of inputs and an output, and each of which is programmable to provide to said output an output signal, and each logic module having means to store an individual program for programming said module, the output signal being any of a plurality of logical functions of said inputs, a first of said logic modules including means for selectively receiving the output signal of a second of said logic modules, wherein a first of said plurality of inputs of said first of said logic modules provides an output signal of said second of said logic modules to said means for selectively receiving, and wherein said first logic module is programmable to provide a modified output signal to said output of said first logic module, said modified output signal being an EXCLUSIVE OR function of the unmodified output signal of said first logic module and said output signal of said second logic module.