I think Pete Donzelli's tree is a slippery elm

For nearly two years, I have been seeing Pete Donzelli's tree there and not knowing what it was.  "How can I figure it out?" I wondered.  Usually The Sibley Guide to Trees is good, but it helps to have a general idea of what kind of tree it might be, so you know what section to look in.  What could this be? I think it looks like an elm.  I turned to the elm section of  The Sibley Guide to Trees, and there it was, a picture of this tree, and it was a slippery elm.  I've known elm for a year and a half, how could I have missed it? Maybe the elms I have known were American elm.