code: public BigInteger optBigInteger(String key, BigInteger defaultValue) {
          Object val = this.opt(key);
          return objectToBigInteger(val, defaultValue);
      }
docs: Get an optional BigInteger associated with a key, or the defaultValue if
  there is no such key or if its value is not a number. If the value is a
  string, an attempt will be made to evaluate it as a number.
  
  @param key
  A key string.
  @param defaultValue
  The default.
  @return An object which is the value.