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Some of Hypothesis's numpy/pandas strategies use a ``fill`` argument to speed
up generating large arrays, by generating a single fill value and sharing that
value among many array slots instead of filling every single slot individually.
When no ``fill`` argument is provided, Hypothesis tries to detect whether it is
OK to automatically use the ``elements`` argument as a fill strategy, so that
it can still use the faster approach.
This patch fixes a bug that would cause that optimization to trigger in some
cases where it isn't 100% guaranteed to be OK.
If this makes some of your numpy/pandas tests run more slowly, try adding an
explicit ``fill`` argument to the relevant strategies to ensure that Hypothesis
always uses the faster approach.