Exotic 2D array element referencing

I saw an exotic array element referencing in a C++ example:

int mat[2][2] = {{1,2},{3,5}};
cout << 0[1[mat]]; //<- what's this?
// it means:
// cout << mat[1][0];

Where is defined this syntax? I didn't learn it, nowhere in C and C++ books.

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