#include <stdio.h>
#include <vector>
#include <set>
#include <queue>
#include <map>
using namespace std;

int N, L, B, F, j, k, i, c, t;
set<int> d[1001];
set<int> start, end;
set<int>::iterator it;
queue<pair<int, int> > q;
long long m[1001];
map<int, long long>::iterator itm;
long long ret;

int main() {

	while (scanf("%d%d%d%d", &N, &L, &B, &F), N > 0) {
		start.clear();
		end.clear();
		for (i = 0; i < N; i++) {
			m[i+1] = 0;
			scanf("%d%d", &j, &k);
			d[j].clear();
			while (k-- > 0) {
				scanf("%d", &c);
				d[j].insert(c);
			}
		}
		for (i = 0; i < B; i++) {
			scanf("%d", &c);
			start.insert(c);
		}
		for (i = 0; i < F; i++) {
			scanf("%d", &c);
			end.insert(c);
		}
		for (it = start.begin(); it != start.end(); ++it) {
			m[*it] = 1;
			q.push(make_pair(1, *it));
		}

		ret = 0;
		while (!q.empty()) {
			k = q.front().first;
			t = q.front().second;
			q.pop();
			if (k == L && end.find(t) != end.end()) ret += m[k];
			for (it = d[t].begin(); it != d[t].end(); ++it) {
				m[*it] += m[t];
				q.push(make_pair(k+1, *it));
			}
		}
		if (ret == 0) printf("impossible\n");
		else printf("%lld\n", ret);
	}
	return 0;
}