You should probably read section 3.2 then as well:

   3.2 Absolute Pathname

   A pathname beginning with a single or more than two slashes;
   see also Pathname.

Note that a pathname starting with exactly two slashes is *not*
an absolute pathname according to Posix.  And 4.11 (Pathname
Resolution) says:

   A pathname that begins with two successive slashes may be
   interpreted in an implementation-defined manner, although
   more than two leading slashes shall be treated as a single
   slash.

Posix has this rule to accomodate DomainOS, which was a Unix-
like OS from Apollo, where paths on the form "//foo/bar/gazonk"
meant the file "/bar/gazonk" on the machine named "foo".  You may
recognize this format from URLs, or MS Windows SMB paths (with
backslashes instead of forward slashes).