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

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

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

    return this.tree.select(this.solvedPath.slice(0, -1).concat(0));
  }
Method returning the leftmost sibling node of the cursor if this one is
pointing at a list.

@return {Baobab} - The leftmost sibling cursor.