(*#*
A subset $P$#<I>P</I># is %\emph{open}%#<I>open</I># if for all $x$#<I>x</I># in $P$#<I>P</I># there exists an open sphere
with centre $x$#<I>x</I># that is contained in $P$#<I>P</I>#.
*)
(*#*
A subset $P$#<I>P</I># is %\emph{open}%#<I>open</I># if for all $x$#<I>x</I># in $P$#<I>P</I># there exists an open sphere
with centre $x$#<I>x</I># that is contained in $P$#<I>P</I>#.
*)