code: String[] split(String line, int assertLen, String usage) {
      String[] ret = split(line);
  
      if (ret.length != assertLen) {
        error(usage);
        return null;
      }
  
      return ret;
    }
docs: Split the line based on spaces, asserting that the number of words is
  correct.
  
  @param line      the line to split
  @param assertLen the number of words to assure
  @param usage     the message to output if there are an incorrect number of
  words.
  @return the split lines, or null if the assertion failed.