\begin{frame}
\small

\begin{block}{Remark}
  \smallskip
  \begin{itemize}
  \item
  Information about \alert<1>{position} is needed to determine descendants:
  \[
  \GREEN{\begin{array}[t]{@{}r@{~}c@{~}l}
  \m{f}(x) & \to & x
  \end{array}
  \qquad\qquad
  A: \m{f}(\m{f}(x)) \xrightarrow[\m{f}(x)\to x]{\alert<1>{?}} \m{f}(x)}
  \]
  Is $1 \project A = \epsilon$ or $1 \project A = \varnothing$?
  \item<2->
  Information about \alert<2>{rewrite rule} is needed to determine
  descendants:
  \[
  \GREEN{\begin{array}[t]{@{}r@{~}c@{~}l}
  \m{f}(x) & \to & \m{f}(x) \\[.5ex]
  \m{f(a)} & \to & \m{f(a)}
  \end{array}
  \qquad\qquad
  A: \m{f(a)} \xrightarrow[\alert<2>{\makebox[5mm]{$?$}}]{\epsilon} \m{f(a)}}
  \]
  Is $1 \project A = 1$ or $1 \project A = \varnothing$?
  \end{itemize}
  \vspace{-2ex}
  \end{block}
\end{frame}