EWD249 - 20

digit represented, because otherwise it was so very improbable that the value true would ever be computed. We were thinking of converging iterations that could oscillate within rounding accuracy. While we had been generous (with the best of intentions!) in regarding real numbers as equal, it quickly turned out that the chosen operation was so weak as to be hardly of any use at all. What it boiled down to was that the established truth of a = b and b = c did not allow the programmer to conclude the truth of a = c . The decision was quickly changed. It is since that experience that I know that the programmer can only use his tool by virtue of (a number of) its properties; conversely, the programmer must be able to state which properties he requires. (Usually programmers don't do so because, for lack of tradition as to what properties can be taken for granted, this would require more explicitness than is otherwise desirable. The proliferation of machines with lousy floating-point hardware -together with the misapprehension that the automatic computer is primarily the tool of the numerical analyst- has done much harm to the profession!)