View Compilation Error

0_0_16972559_23262.cpp: In function 'is juzhenmul(is, is)':
0_0_16972559_23262.cpp:32:22: error: no match for 'operator[]' (operand types are 'is' and 'long long int')
         ans.a[i][t]=a[i][t]+(a.a[i][j]*b.a[j][t])%(p-1);
                      ^