-Some of the classes shown here (the ones sitting in white boxes) are
-abstract base classes that are of no interest at all for the user. They
-are used internally in order to avoid code duplication if two or more
-classes derived from them share certain features. An example would be
-@code{expairseq}, which is a container for a sequence of pairs each
-consisting of one expression and a number (@code{numeric}). What
-@emph{is} visible to the user are the derived classes @code{add} and
-@code{mul}, representing sums of terms and products, respectively.
-@xref{Internal Structures}, where these two classes are described in
-more detail.
-
-At this point, we only summarize what kind of mathematical objects are
-stored in the different classes in above diagram in order to give you a
-overview:
+The abstract classes shown here (the ones without drop-shadow) are of no
+interest for the user. They are used internally in order to avoid code
+duplication if two or more classes derived from them share certain
+features. An example is @code{expairseq}, a container for a sequence of
+pairs each consisting of one expression and a number (@code{numeric}).
+What @emph{is} visible to the user are the derived classes @code{add}
+and @code{mul}, representing sums and products. @xref{Internal
+Structures}, where these two classes are described in more detail. The
+following table shortly summarizes what kinds of mathematical objects
+are stored in the different classes: