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