"It's a beautiful London plane," I said.
"It's not a plane tree," the man replied. "It's an acer, I think. Perhaps a sycamore?"
"I think it's a London plane," I repeated, having researched the matter.
"No it's not," his wife insisted. "When we meet next, we'll tell you what it actually is."
I considered killing them instantly, but relented. Instead I called the police, who duly arrested them: misidentifying a tree is a criminal offence in Scotland. They were each issued with fixed-penalty fines of £200.