left() {
    checkPossibilityOfDynamicTraversal('left', this.solvedPath);

    const last = +this.solvedPath[this.solvedPath.length - 1];

    if (isNaN(last))
      throw Error('Baobab.Cursor.left: cannot go left on a non-list type.');

    return last ?
      this.tree.select(this.solvedPath.slice(0, -1).concat(last - 1)) :
      null;
  }
Method returning the left sibling node of the cursor if this one is
pointing at a list. Returns `null` if this cursor is already leftmost.

@return {Baobab} - The left sibling cursor.