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Determine
          if an execution_context
          contains a specified service type.
        
template< typename Service> bool has_service( execution_context & e);
          This function is used to determine whether the execution_context contains a service
          object corresponding to the given service type.
        
                The execution_context object
                that owns the service.
              
          A boolean indicating whether the execution_context contains the
          service.
        
          Header: asio/execution_context.hpp
        
          Convenience header: asio/execution.hpp