$\mathscr{E} = \left( \dfrac{a-1}{3a + (a-1)^2} - \dfrac{1 - 3a + a^2}{a^3 - 1} - \dfrac{1}{a-1} \right) \div \dfrac{a^2 + 1}{1-a}$

Let $a = 2016$. If $\mathscr{E}$ can be expressed in the form $\dfrac{x}{y}$, where $x$ and $y$ are coprime positive integers, find $x+y$.