View Compilation Error

0_0_21390163_11514.cpp: In member function 'node node::operator*(const node&) const':
0_0_21390163_11514.cpp:37:30: error: 'j' was not declared in this scope
                     ans.f[i][j] = (ans.f[i][j] + (f[i][k] * x.f[k][j] + mod) % mod) % mod;
                              ^