\newcommand{\TranShocktpOne }{\Theta_{t+1}} % \Theta_{t+1} includes the aggregate transitory component
\begin{eqnarray}
  \label{eq:ell}
\ifnumber  \pmb{\ell}_{t+1} & = & \overbrace{\theta_{t+1}\TranShocktpOne}^{\equiv \pmb{\theta}_{t+1}}\underbrace{{p}_{t+1} {P}_{t+1}}_{\equiv \mathbf{p}_{t+1}}
\end{eqnarray}
