equation number and references with overlays

hello,

i think that this problem has been addressed before, but i still have not managed to find a satisfactory solution.

i have a number of equations in a series of overlays. in later overlays i wish to refer to equations which came up previously. it seems that i can only refer to equations from the first overlay.

for example:

\overlays{4}{%
  \begin{slide}{bli}
    \onlySlide*{1}{
      \begin{equation}
        x = y
        \label{eq:a}
      \end{equation}
      Eq \ref{eq:a} Eq \ref{eq:b} Eq \ref{eq:c} Eq \ref{eq:d}
    }
    \onlySlide*{2}{
      \begin{equation}
        x = y
        \label{eq:b}
      \end{equation}
      Eq \ref{eq:a} Eq \ref{eq:b} Eq \ref{eq:c} Eq \ref{eq:d}
    }
    \onlySlide*{3}{
      \begin{equation}
        x = y
        \label{eq:c}
      \end{equation}
      Eq \ref{eq:a} Eq \ref{eq:b} Eq \ref{eq:c} Eq \ref{eq:d}
    }
    \onlySlide*{4}{
      \begin{equation}
        x = y
        \label{eq:d}
      \end{equation}
      Eq \ref{eq:a} Eq \ref{eq:b} Eq \ref{eq:c} Eq \ref{eq:d}
    }
  \end{slide}
}

here only the reference to {eq:a} works. the reset are "undefined".

does anyone know how to deal with this situation?

thanks!
andrew.


-- 
Andrew B. Collier

Antarctic Research Fellow                                   tel: +27 31 2601157
Space Physics Research Institute                            fax: +27 31 2616550
University of KwaZulu-Natal, Durban, 4041, South Africa


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