A correction to EWD1240

A correction of EWD1240

The example, introduced on p. EWD1240-1, where "E" is defined to mean "lies inside" for circles in the plane, is wrong. The error emerges on p. EWD1240-12 , where it is suggested that $x \equiv z \land y \equiv z$ ,i.e. for $z$ : encompassing both $x$ and $y$ ,is equivai- lent to ${x}^{!}y \leq z$ ,i.e. encompassing a third circle denoted by ${x}^{!}y$ . Here is

the counterexample: the dotted circle encompasses x and y, but not the smallest circle encompassing them.

The error can be remedied by defining E, "lies inside" for convex curves instead of for circles.

I owe Rutger M. Dijkstra my gratitude for pointing out this error.

Austin, 5 May 1997

prof. dr. Edsger W. Dgksta Department of Computer Sciences The University of Texas at Austin Austin, TX 78712 - 1188 USA