+@cindex branch cut
+For functions that have a branch cut in the complex plane GiNaC follows
+the conventions for C++ as defined in the ANSI standard. In particular:
+the natural logarithm (@code{log}) and the square root (@code{sqrt})
+both have their branch cuts running along the negative real axis where
+the points on the axis itself belong to the upper part.
+
+Besides evaluation most of these functions allow differentiation, series
+expansion and so on. Read the next chapter in order to learn more about
+this.
+
+
+@node Relations, Important Algorithms, Built-in functions, Basic Concepts
+@c node-name, next, previous, up
+@section Relations
+@cindex relations (class @code{relational})
+
+Sometimes, a relation holding between two expressions must be stored
+somehow. The class @code{relational} is a convenient container for such
+purposes. A relation is by definition a container for two @code{ex} and
+a relation between them that signals equality, inequality and so on.
+They are created by simply using the C++ operators @code{==}, @code{!=},
+@code{<}, @code{<=}, @code{>} and @code{>=} between two expressions.