add(key, value) {
        const length = key.length;
        if (length === 0) {
          throw new TypeError("Unreachable");
        }
        let index = 0;
        let node = this;
        while (true) {
          const code = key.charCodeAt(index);
          if (code > 127) {
            throw new TypeError("key must be ascii string");
          }
          if (node.code === code) {
            if (length === ++index) {
              node.value = value;
              break;
            } else if (node.middle !== null) {
              node = node.middle;
            } else {
              node.middle = new _TstNode(key, value, index);
              break;
            }
          } else if (node.code < code) {
            if (node.left !== null) {
              node = node.left;
            } else {
              node.left = new _TstNode(key, value, index);
              break;
            }
          } else if (node.right !== null) {
            node = node.right;
          } else {
            node.right = new _TstNode(key, value, index);
            break;
          }
        }
      }
@param {string} key
@param {any} value