Some numeric operations can result in a value represented by the constant NAN.
This result represents an undefined or unrepresentable value in floating-point calculations.
Any loose or strict comparisons of this value against any other value, including itself, but except TRUE, will have a result of FALSE.
Because NAN represents any number of different values, NAN should not be compared to other values, including itself, and instead should be checked for using is_nan().
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