If p.has_root_directory(),
then removes any root directory and relative path
from the generic format pathname.
Otherwise,
if !has_root_directory()&& is_absolute() is true
or if has_filename() is true,
then appends path::preferred_separator to the generic format pathname.
Even if //host is interpreted as a root-name,
both of the paths path("//host")/"foo" and path("//host/")/"foo"
equal "//host/foo" (although the former might use backslash as the
preferred separator).