This function is inspired by SQL function
NVL, and simply returns
the first argument that is not
NULL if all arguments are NULL.
Expressions to be tested.
Note that an earlier version of this function took only two arguments:
EXPR, that was tested and returned if not
NULLV, which was returned if
NULL. The new version produces identical output for the same
(two-argument) input, but tests any number of expressions
The first argument that is not
NULL if all
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