void parser (  list<node *> & input ) {
    
    list<node *>::iterator i = input.begin(), i1, o ;
    char const * op [] = {"*","/","-","+"} ;
    list<string> operators(op,op+sizeof(op)/sizeof(op[0])) ;
    list<string>::iterator str_i = operators.begin() ;
    int state = state::binary ;
    
       while ( state != state::quit ) {
            
            
            switch (state) {
                
                case state::binary : 
                                        if ( i != input.end() && ((*i)->type == state::value || (*i)->type == state::expression) ) {
                                            i1 = i ;
                                            ++i ;
                                            if ( i != input.end() && ((*i)->type == state::operation || (*i)->type == state::expression) && (*i)->token == *str_i ) {
                                                node *o = *i ;
                                                ++i ;
                                                if ( i != input.end() && ((*i)->type == state::value  || (*i)->type == state::expression) ) {
                                                    o->left = *i1 ;
                                                    o->right = *i;
                                                    o->type = state::expression ;
                                                    input.erase(i1) ;
                                                    input.erase( find(input.begin(), input.end(), o->right ) ) ;
                                                    i = input.begin() ;
                                                    state = state::binary ;
                                                }
                                            }
                                        }
                                        else if ( i != input.end() ) {
                                            state = state::binary ;
                                            ++i ;
                                        }
                                        else if ( i == input.end() && str_i != operators.end() ) {
                                            i = input.begin() ;
                                            state = state::binary ;
                                            ++str_i ;
                                            
                                        }
                                        else
                                            state = state::quit ;
                                        
                                        break ;
            }
        }
    }