After 5 minutes of staring at it: Its typesystem sudoku. Each row and each col in the grid must add up to 15 (T<>), bit each number in the grid must be different (Df<>).
Grid will only be a type alias for the value true (google “Dependent types”) only if all Type Parameters (wich are values) hold up to the Sudoku conditions).
The file would not compile with “true as Grid” when grid type-aliases to false.