Is there some way to repress creation of a bubble displayed in the Frankfurt progress header for specific frames? In the example below, when compiled it will show four bubbles on the top of the ticker. Instead I'd like for there to only be two bubbles one for Slide 1 and another for Slide 2a, but to suppress bubbles for slides 2b and 2c.
\documentclass[pdf]{beamer}
\mode<presentation>{\usetheme{Frankfurt}}
\begin{document}
\section{Intro}
\subsection{}
\begin{frame}{Slide1}
Text
\end{frame}
\subsection{}
\begin{frame}{Slide2a}
Image
\end{frame}
\subsection{}
\begin{frame}{Slide2b}
Image
\end{frame}
\subsection{}
\begin{frame}{Slide2c}
Image
\end{frame}
\end{document}