+ mv_has_value()).
+
+ We remove the first value from MV, not the last, because the
+ common use for this function is in iterating through a set of
+ missing values. If we remove the last value then we'll output
+ the missing values in order opposite of that in which they
+ were added, so that a GET followed by a SAVE would reverse the
+ order of missing values in the system file, a weird effect. */