[see also: view, regard, consider]
If one thinks of $x, y$ as space variables and of $z$ as time, then ......
It may help to think of $F$ as being a smooth approximation to the Heaviside function.
This result may be thought of as a sort of regularity theorem.
So $s$ can be thought of as $q$ with $F^q$ extended but $X^q$ left the same.
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