... subroutine¹

We thank M. Seymour for pointing out an error in the previous version of this subroutine and for providing us with a corrected version of it.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.