code: public static String STVtoID(String source,
                                   String target,
                                   String variant) {
          StringBuilder id = new StringBuilder(source);
          if (id.length() == 0) {
              id.append(ANY);
          }
          id.append(TARGET_SEP).append(target);
          if (variant != null && variant.length() != 0) {
              id.append(VARIANT_SEP).append(variant);
          }
          return id.toString();
      }
docs: Given source, target, and variant strings, concatenate them into a
  full ID.  If the source is empty, then "Any" will be used for the
  source, so the ID will always be of the form s-t/v or s-t.