#include <iostream>
using std::cout;
using std::endl;

struct T {
    T*prev;
    T*next;
    int seq;
};

void weed_list( T* &head, bool (*condition)(T*) ) {
    for ( T** dp = &head ; *dp ; ) {
        T* c = *dp;
        T* *n = &c->next;
        if (condition(c)) {
            if (*n)
                (*n)->prev = c->prev;
            *dp = *n;
            cout << "deleting " << c->seq << " " << (long)c << endl;
        } else {
            dp = n;
        }
    }
}

extern T a,b,c,d,e;

T a = { 0,  &b, 0 };
T b = { &a, &c, 1 };
T c = { &b, &d, 2 };
T d = { &c, &e, 3 };
T e = { &d, 0,  4 };

T* h = &a;

bool prune(T*c) {
    return c->seq % 3 == 0;
}

int main() {
    cout << "starting with..." << endl;
    for (T* c = h; c;c=c->next)
        cout << "starter " << c->seq << " " << (long)c << endl;
    cout << endl;
    cout << "pruning..." << endl;
    weed_list(h, prune);
    cout << endl;
    cout << "keeping..." << endl;
    for (T* c = h; c;c=c->next)
        cout << "keeping " << c->seq << " " << (long)c << endl;
}