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vector<int> pivots; // ith pivot is in ith row
void gauss(int n, int m) {
for (int r = 0, c = 0; c < m; c++) {
for (int i = r; i < n; i++) {
if (mat[i][c] != 0){
swap(mat[r], mat[i]);
break;
}}
if (mat[r][c] == 0) continue;
ll f = multInv(mat[r][c], mod);
for (ll& x : mat[r]) x = x * f % mod;
for (int i = 0; i < n; i++) {
if (i == r) continue;
f = mat[i][c];
for (int j = c; j < m; j++) {
mat[i][j] = (mat[i][j] - f * mat[r][j] % mod + mod) % mod;
}}
pivots.push_back(c);
if (++r == n) break;
}} // no solution if pivots.back() == m-1
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