* the special variant NO_VARIANT is stored in slot zero of the UVector of variants.
*/
private void registerSTV(final String source, final String target, final String variant) {
// assert(source.length() > 0);
// assert(target.length() > 0);
CaseInsensitiveString cisrc = new CaseInsensitiveString(source);
CaseInsensitiveString citrg = new CaseInsensitiveString(target);
CaseInsensitiveString civar = new CaseInsensitiveString(variant);
Map<CaseInsensitiveString, List<CaseInsensitiveString>> targets = specDAG.get(cisrc);
if (targets == null) {
targets = Collections.synchronizedMap(new HashMap<CaseInsensitiveString, List<CaseInsensitiveString>>());
specDAG.put(cisrc, targets);
}