_set(id, value, opId) {
    if (Object.isFrozen(this.entries)) {
      throw new Error('A table can only be modified in a change function')
    }
    if (isObject(value) && !Array.isArray(value)) {
      Object.defineProperty(value, 'id', {value: id, enumerable: true})
    }
    this.entries[id] = value
    this.opIds[id] = opId
  }
Sets the entry with key `id` to `value`. `opId` is the ID of the operation
performing this assignment. This method is for internal use only; it is
not part of the public API of Automerge.Table.