Well, relativeness essentially depends on the scheme's syntax, for data URIs this is undefined. I wanted to return a boolean so I made the arbitrary decision that `pf:is-absolute` systematically looks at the path component of the URI, whether it is defined (as in hierarchical `file` or `http` URIs) or not (as in opaque `data` URIs).

IMHO returning `false` on URIs were relativeness is undefined is a reasonable default. WDYT ?