Description:
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Creating a private method __clone, while cloning $this, where $this is an instance of a child class of self, will set the context of the caller to $this, instead of self, which is somewhat unusual, given that calling a private method from self is otherwise succesful.

Making the method protected solves the issue, but makes cloning by other classes in the hierarchy possible too, which may be undesirable.

Test script:
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This is an example:

http://pastebin.com/5z4xrQdR

Expected result:
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four times the string "I occurred"

Actual result:
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three times the string "I occurred" and an error message

Fatal error: Call to private F::__clone() from context 'E' in __FILE__ on line 48