This is a bit picky, but section 4.1.2 contains a series of rules like
this one:
o metalink:file elements MAY contain exactly one metalink:copyright
element.
Note that (if parsed rigorously according to RFC2119) this does not forbid an
element that contains more than one metalink:copyright element. To be rigorous,
you'd have to add "and MUST NOT contain more than one such element".