NAME

  Math::Expr::Num - A node in the expretion tree, used as superclass only

SYNOPSIS

  package Math::Expr::Num;
  require Math::Expr::Node;

  use Math::Expr::Node;
  use vars qw(@ISA);
  @ISA = qw(Math::Expr::Node);

DESCRIPTION

  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.