Moving boxes with \step[...][...]

Hi,

I am trying to achieve the following effect:

On one slide, under a common heading, I first want to display a simple
Scheme and an explaining Figure below. In the next step, the scheme is
replaced by a more complicated one which also takes more vertical
space. The Figure that explains Scheme 2 is different, but has exactly
the same dimensions, and should be displayed at the same place where
Figure 1 was. In a third step, a conclusion text appears below Scheme 2
and figure 2.

The problem, as can be seen in the minimal example below, is that the
figures hop around: It is placed lower with the flatter Scheme 1.

How can I prevent that, or how could i achieve the same with different
commands?=20

Regards, Frank

\documentclass[display]{powersem}
\usepackage{texpower}

\begin{document}
\begin{slide}[27.5cm,20cm]
\section{Foldon as a model for folding \emph{and} association}

\stepwise*{
  \begin{center}
    \steponce{\fbox{\rule{0cm}{0.5cm}\rule{8cm}{0cm}}} % Scheme 1
    \bstep{\fbox{\rule{0cm}{2cm}\rule{8cm}{0cm}}} % Scheme 2, slightly deep=
er
  \end{center}

  \begin{center}%
    \step[\value{step}=3D1][\value{step}=3D1]{%
      \fbox{\rule{0cm}{3cm}\rule{3cm}{0cm}} % Figure 1
  }%
  \step[\value{step}=3D2][\value{step}>1]{%
      \fbox{\rule{0cm}{3cm}\rule{3cm}{0cm}} % Figure 2, same
                                % dimensions as Figure 1=20
  }

  \step[\value{step}=3D3][\value{step}=3D3]{
      Exchange prolines in order to get simpler kinetics
  }
\end{center}
}

\end{slide}
\end{document}

--=20
Frank K=FCster, Biozentrum der Univ. Basel
Abt. Biophysikalische Chemie



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