docs: public void put(final String pattern, final V value) throws IllegalStateException {
  
          if (pattern != null) {
              if (keys.containsKey(pattern)) {
                  throw new IllegalStateException("");
              }
              keys.put(pattern, value);
  
              final Pattern p = Pattern.compile(pattern);
  
              patternMap.put(p, value);
          } else {
              nullValue = value;
          }
      }