code: private int findFirstPluralNumberArg(int msgStart, String argName) {
          for(int i=msgStart+1;; ++i) {
              Part part=msgPattern.getPart(i);
              Part.Type type=part.getType();
              if(type==Part.Type.MSG_LIMIT) {
                  return 0;
              }
              if(type==Part.Type.REPLACE_NUMBER) {
                  return -1;
              }
              if(type==Part.Type.ARG_START) {
                  ArgType argType=part.getArgType();
                  if(argName.length()!=0 && (argType==ArgType.NONE || argType==ArgType.SIMPLE)) {
                      part=msgPattern.getPart(i+1);  // ARG_NUMBER or ARG_NAME
                      if(msgPattern.partSubstringMatches(part, argName)) {
                          return i;
                      }
                  }
                  i=msgPattern.getLimitPartIndex(i);
              }
          }
      }
docs: Returns the ARG_START index of the first occurrence of the plural number in a sub-message.
  Returns -1 if it is a REPLACE_NUMBER.
  Returns 0 if there is neither.