```
wc -l test/unit/math/prim/mat/fun/*.cpp | sort -h | head
```

These files are basically empty until you get to `diag_matrix_test.cpp`

:

```
2 test/unit/math/prim/mat/fun/columns_dot_product_test.cpp
2 test/unit/math/prim/mat/fun/Eigen_test.cpp
2 test/unit/math/prim/mat/fun/log_determinant_test.cpp
2 test/unit/math/prim/mat/fun/mdivide_left_tri_low_test.cpp
2 test/unit/math/prim/mat/fun/mdivide_right_tri_low_test.cpp
2 test/unit/math/prim/mat/fun/rows_dot_product_test.cpp
3 test/unit/math/prim/mat/fun/dot_product_test.cpp
3 test/unit/math/prim/mat/fun/LDLT_factor_test.cpp
3 test/unit/math/prim/mat/fun/typedefs_test.cpp
9 test/unit/math/prim/mat/fun/diag_matrix_test.cpp
```