add (key, value) {
    const length = key.length
    if (length === 0) {
      throw new TypeError('Unreachable')
    }
    let index = 0
    /**
     * @type {TstNode}
     */
    let node = this
    while (true) {
      const code = key.charCodeAt(index)
      // check code is ascii string
      if (code > 0x7F) {
        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
@returns {void}