[segway] forward declaration

from http://www-subatech.in2p3.fr/~photons/
subatech/soft/carnac/CPP-INC-1.shtml
1. B only uses references or pointers to A. Use forward declaration then : you don't need to include . This will in turn speed a little bit the compilation.

  class A ;
  
  class B {
    private:
      A* fPtrA ;
    public:
      void mymethod(const& A) const ;
  } ;

2. B derives from A or B explicitely (or implicitely) uses objects of class A. You then need to include

  #include 
  
  class B : public A {
  
  } ;
  
  class C {
    private:
      A fA ;
    public:
      void mymethod(A par) ;   
  }