Each expretion is represented by a tree where each opperation and variable
is a separate node. This class contain the common code for all those noeds.
It also defines all the common methods used in those node classes and does
some typecheckinig for them. Therefor the methodname in the subclasses
should start with a '_'-char followed by the actually method name. This
method will be called by the acutall method in this class after the
typecheking is done.