private static SquareMatrix <T> Add <T>(SymmetricMatrix <T> lhs, SquareMatrix <T> rhs) => Add(rhs, lhs);
private static SymmetricMatrix <T> Add <T>(SymmetricMatrix <T> lhs, DiagonalMatrix <T> rhs) => Add(rhs, lhs);
private static SquareMatrix <T> Add <T>(SymmetricMatrix <T> lhs, SquareMatrix <T> rhs) => Add(rhs, lhs);
private static SymmetricMatrix <T> Add <T>(SymmetricMatrix <T> lhs, DiagonalMatrix <T> rhs) => Add(rhs, lhs);