Alder Buckthorn from the subconscious

[bunn]

I took a photo of this a few weeks ago because I didn't recognise it. I expect to recognise trees - at least, trees growing in a context that says 'I am a native tree!' like this one.  Later, I found the photo and thought 'what was that tree anyway?'  So I typed things vaguely into Google, but had no luck.  Then the word 'Alder'  came into my head, and then the word 'buckthorn'. And so it appears to be.   Yet I would have sworn I did not know that tree.

 Bad brain, no biscuit.