Description:
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A generator can be rewound if and only if its first yielded value has been fetched. As soon as we fetch the second (or any further) yielded value and *then* try to rewind it we actually get the exception. This is inconsistent.

Test script:
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$f= function() { yield 1; yield 2; yield 3; };
$gen= $f();

foreach ($gen as $val) { var_dump($val); }
foreach ($gen as $val) { var_dump($val); }  // Exception("Cannot rewind a generator that was already run")

# Here's the bug:
foreach ($gen as $val) { var_dump($val); break; }
foreach ($gen as $val) { var_dump($val); }  // No exception

Expected result:
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int(1)
Exception("Cannot rewind a generator that was already run")


Actual result:
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int(1)
int(1)
int(2)
int(3)