namespace trial {
  namespace dynamic {
    template<typename Visitor, template< typename > class Allocator> 
      auto visit(Visitor &&, basic_variable< Allocator > &);

    // Immutable visitation. 
    template<typename Visitor, template< typename > class Allocator> 
      auto visit(Visitor && visitor, 
                 const basic_variable< Allocator > & variable);
  }
}