In the Matrix class constructor, you set the default for the content parameter like this:

constructor(width, height, content = () => undefined) (i.e. no parameters in the function)

But then in theSymmetricMatrix class constructor, set the default for the content parameter like this:

constructor(size, content = (x, y) => undefined) (i.e. x and y parameters in the function)

Is there a reason these are different?