code: public static String getRequiredMapParam(Map<String, String[]> queryMap,
  			String field) throws BadQueryException {
  		// TODO: Throw something different, org.archive.wayback.util should have
  		// no references outside of org.archive.wayback.util 
  		String value = getMapParam(queryMap, field);
  		if (value == null) {
  			throw new BadQueryException("missing field " + field);
  		}
  		if (value.length() == 0) {
  			throw new BadQueryException("empty field " + field);
  		}
  		return value;
  	}
docs: Extract the first value in the array mapped to by field in queryMap
  @param queryMap the Map in which to search
  @param field the field value desired
  @return the first value in the array mapped to field in queryMap, if
  present. A BadQueryException is thrown if there is no appropriate value
  @throws BadQueryException if there is nothing mapped to field, or if the
  Array mapped to field is empty