code: void put(ChronoElement<?> element, Object v) {
  
          if (element == null) {
              throw new NullPointerException();
          }
  
          if (v == null) { // removal
              if (this.map != null) {
                  this.map.remove(element);
                  if (this.map.isEmpty()) {
                      this.map = null;
                  }
              }
          } else {
              Map<ChronoElement<?>, Object> m = this.map;
              if (m == null) {
                  m = new HashMap<>();
                  this.map = m;
              }
              m.put(element, v);
          }
  
      }
docs: called by format processors