template<typename iterator>void parse (iterator begin,iterator end,list<node *> & output) {
    string::iterator i = begin, i1 ;
    while (i != end) {
        if (*i >= '0' && i* <= '9' && i != end) {
            i1 = i ;
            while (*i >= '0' && i* <= '9' && i != end) {
                ++i ;
            }
            node* n = new node ;
            string str1 (i1,i-1) ;
            istringstream stream (str1) ;
            float f ;
            stream >> f ;
            n->value = f ;
            output.push_back(n) ;
        }
        else
            if ( isalpha(*i) && i != end ) {
                i1 = i ;
                while ( isalpha(*i) && i != end ) {
                    ++i ;
                }
                node* n = new node ;
                string str1 (i1,i-1) ;
                n->operation = find_op(str1.c_str()) ;
                output.push_back(n) ;
            }
        else    
            if ( (*i == '+') || (*i == '-') || (*i == '*') || (*i == '/') || (*i == '^') && i != end ) {
                node* n = new node ;
                n->operation = find_op(*i) ;
                output.push_back(n) ; 
                ++i ;
            }
    }
            
}