Parenthesization/Examples/4

Example of Parenthesization
A word of $4$ elements can be parenthesized in $5$ distinct ways:
 * $\quad a_1 \left({a_2 \left({a_3 a_4}\right)}\right)$
 * $\quad a_1 \left({\left({a_2 a_3}\right) a_4}\right)$
 * $\quad \left({a_1 a_2}\right) \left({a_3 a_4}\right)$
 * $\quad \left({a_1 \left({a_2 a_3}\right)}\right) a_4$
 * $\quad \left({\left({a_1 a_2}\right) a_3}\right) a_4$