Q:

Beamer frame with two columns and sequential uncover

I am trying to build a frame with 2 column.  I have overlays in both columns and I want then to be displayed sequentially.  I mean, first all the contents of first column (with the overlays) and then the contents of second column (also with its overlays).  Here is the MWE:
\documentclass{beamer}

\usepackage{arev}

\begin{document}

\begin{frame}{test}

\begin{columns}[t]

\column{0.5\textwidth}

\begin{center}
\rule{1cm}{1cm}
\only<2->{\vskip2pt \rule{1cm}{1cm}}
\only<3->{\vskip2pt \rule{1cm}{1cm}}
\end{center}

\column{0.5\textwidth}

\begin{itemize}
\item<4-> aaa
\item<5-> bbb
\item<6-> ccc
\end{itemize}

\end{columns}

\end{frame}

\end{document}

To be specific, I want the structure:
\begin{itemize}
\item<4-> aaa
\item<5-> bbb
\item<6-> ccc
\end{itemize}

To be replaced with something like:
\begin{itemize}[<+->]
\item aaa
\item bbb
\item ccc
\end{itemize}

I tested various approaches with \only<>{} \visible<>{} but nothing worked. 

A:

One possibility:
\documentclass{beamer}
\usepackage{arev}

\begin{document}

\begin{frame}{test}
\begin{columns}[t]
\column{0.5\textwidth}
\begin{center}
\rule{1cm}{1cm}
\only<2->{\vskip2pt \rule{1cm}{1cm}}
\only<3->{\vskip2pt \rule{1cm}{1cm}}
\end{center}
\column{0.5\textwidth}
\begin{itemize}[<+(3)->]
\item aaa
\item bbb
\item ccc
\end{itemize}
\end{columns}
\end{frame}

\end{document}

The syntax +(<number>) adds <number> to the value of the counter beamerpauses. 
This is even more automated since you don't need to manually control counters:
\documentclass{beamer}
\usepackage{arev}

\begin{document}

\begin{frame}{test}
\begin{columns}[t]
\column{0.5\textwidth}
\begin{center}
\onslide<+->{\rule{1cm}{1cm}}
\onslide<+->{\vskip2pt \rule{1cm}{1cm}}
\onslide<+->{\vskip2pt \rule{1cm}{1cm}}
\end{center}
\column{0.5\textwidth}
\begin{itemize}[<+->]
\item aaa
\item bbb
\item ccc
\end{itemize}
\end{columns}
\end{frame}

\end{document}

Or using \only instead of \onslide (depending on the intended effect).