public static String prefixFromPlain(String u) {
        u = fromPlain(u);
        // truncate to implied prefix
        u = SurtPrefixSet.asPrefix(u);
        return u;
    }
public static String prefixFromPlain ( String u ) { u = fromPlain ( u ) ; u = SurtPrefixSet . asPrefix ( u ) ; return u ; }
Given a plain URI or hostname/hostname+path, deduce an implied SURT
prefix from it. Results may be unpredictable on strings that cannot
be interpreted as URIs.

UURI 'fixup' is applied to the URI that is built.

@param u URI or almost-URI to consider
@return implied SURT prefix form