node* _build_syntax_tree( list<node*> & output ) {
    
    while ( output.size()>1 ) {
            list<node*>::iterator i = output.begin(), i1 = output.begin() ;
            while (i != output.end()) {
                if ((*i)->operation && (*i)->priority > (*i1)->priority) i1 = i ;
                ++i ;
            }
            switch ((*i1)->operation->type) {
                case function::PREFIX : {
                    (*i1)->priority = -1 ;
                    i=i1;
                    ++i;  
                    (*i1)->kaire = *i ; 
                    output.erase(i) ; 
                    break ;
                }
                case function::INFIX : {
                    node* n = *i1 ;
                    i=i1;
                    ++i;
                    --i1;
                    n->priority = -1 ; 
                    n->kaire = *i1 ; 
                    n->desne = *i ; 
                    output.erase(i1) ;
                    output.erase( find(output.begin(), output.end(), n->desne )) ;
                    break ;
                }
            }
    }
    return *output.begin();
}