 A number of formal techniques to invalidate ECQ have been devised. As the interest in paraconsistent logic grew, different techniques developed in different parts of the world. As a result, the development of the techniques has somewhat a regional flavour (though there are, of course, exceptions, and the regional differences can be over-exaggerated; see Tanaka 2003). Some of these have been summarised in Brown 2002 and Priest 2002. The list of systems canvassed here is by no means exhaustive and will be expanded in future updates.