deleteAt(index, numDelete = 1) {
    if (this.context) {
      this.context.splice(this.path, index, numDelete, [])
    } else if (!this[OBJECT_ID]) {
      this.elems.splice(index, numDelete)
    } else {
      throw new TypeError('Automerge.Text object cannot be modified outside of a change block')
    }
    return this
  }
Deletes `numDelete` list items starting at position `index`.
if `numDelete` is not given, one item is deleted.