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But, I think cherry is sour too. I didn't know what greengage is. And, now I think maybe that tree can be a plum tree. Do the fruit grow to fit in palm of hand?

Plums can to be sour and sweet. I only like them when they are VERY ripe and sweet.

https://www.google.com/search?q=plum+tree&client=firefox-b-m&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiczZH8h6LjAhVWZM0KHRg4B7UQ_AUIBigB&biw=420&bih=618#mhpiv=2&spf=1562476883967

This is a plum tree.

The leaves on your tree don't match leaves on cherry or cranberry tree so, I must be wrong.

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And here is a photo that says It might be red plum or greengage. We don't have greengage in Canada... not even imported to grocery store.

https://www.123rf.com/photo_105203264_red-plums-or-greengage-on-a-plum-tree-bush-.html

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Oh didn't know that it can't be found there!
Anyway, thanks for your time

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Actually it's not palm tree cuz we have one of them and they are different. I mean the tree and it's fruit. I have searched about it and the nearest thing I could find was greengages. Mostly greengages are green and the fruit is sweet but this one is red and fruit is sour and sweet. But still they call it greengages like the green version. In our language, both of them, green and red version called as "گوجه سبز".

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