Making a new class, derivative of gen_user
  
  You must declare
  class my_gen::public gen_user {
    virtual my_gen * memory_alloc() const { return (my_gen *) malloc(sizeof(*this)); }
  };
  and redefine each operator you would like to redefine.
  You must define a constructor taking a gen (it will be of type _COMP most
  certainly) and returning a gen of type _USER with it's _USERptr pointing
  to a * my_gen, make from this constructor a standard unary_function_ptr 
  at_my_gen with name my_gen and register it for the lexer
  sym_tab_register("my_gen",at_my_gen);
  so that the user can write something like that
  a:=my_gen(1,2,3)