\begin{array}{llll}
\text{Top} & 3 & \frac{a + 1}{2} & \overset{\frac{a + 1}{2}}{y} \\
\text{Bottom} & 3 & \frac{a + 1}{2} & \overset{\frac{a + 1}{2}}{y} \\
\text{Center} & 3 & \frac{a + 1}{2} & \overset{\frac{a + 1}{2}}{y} \\
\text{Baseline} & 3 & \frac{a + 1}{2} & \overset{\frac{a + 1}{2}}{y} \\
\end{array}
