It may be possible to understand the Buddha's formula more readily by means of an example based on phenomenal existence.
Take any object - say a jug - and let it represent, be a symbol for, reality. If you then photograph it you have a negative representation of it in two dimensions, composed merely of light and shade. The positive reproduction of that symbol reverses the light and shade, and reveals an image which we can recognise as that of what we know as a jug. An animal, unable to form concepts, cannot normally recognise the object, but sees only light and shade.
That, in fact, is the Buddha's formula, in reverse. The positive image is that which appears to be in phenomenal existence. The negative image is the background of that, its relative reality from which it derives, that which precedes it and without which it cannot be. But both are just two-dimensional images composed of light and shade, quite illusory, unrecognisable except by beings who use concepts - just representations of the jug-reality whose existence is in a further dimension.
So you have the formula exactly: it is (as an appearance); it is not (is a negative): therefore that which is represented (and is real) alone is.
Note 1: We notice in passing that this example reveals clearly the three degrees of perception available to man: perception of 'reality', known only to the awakened; perception of 'relative reality', the objective world known to us; perception of images and symbols by means of conceptualisation. The first is real; the second is a representation of the real; the third is imaginary. The Buddha's formula treats of the two first forms of perception; our example is applied to the two latter.
Note 2: The photographic apparatus represents the sensorial apparatus by means of which we interpret, or create, the apparent world which surrounds us.