Uchiyama Roshi, in his commentary in the book Master Dogen’s Zazen Meditation Handbook.

In mathematics, one plus one equals two. None of us doubt this. But actually, this is only correct from the standpoint of mathematics. In life experience, when my car crashes into another car, the hoods are dented, tires come off, and glass breaks into pieces. No cars remain. With cars, one plus one makes zero, or we could say that one plus one makes infinity, because the two cars break apart into an infinite number of pieces.

The following example is even clearer. One man plus one woman makes three or four when they have children.

Once I took a piss in the ocean. At that moment I clearly understood that one plus one makes only one. The one was the ocean. Nothing was changed even after my piss was added. The ocean was not concerned about my piss at all.

:-)