insertCharsAt(char, value, attributes) {
    let ins = {}
    ins[char.id] = {
      value: value,
      attributes: attributes
    }
    this.insert(ins)
  }
Insert chars with optional attributes at a given position.
@param {Char} char The position at which to insert.
@param {string} value The string value to insert.
@param {object} [attributes] Attributes to set, or no attributes if not set. The attributes are
  cloned before setting so that they cannot be modified by simply changing the object reference.
  This type of change would not propagate through the replica.