   FIELD(str,str1,str2,str3,...)
Returns the index of str in the str1, str2,
str3, ... list.
Returns 0 if str is not found.

If all arguments to FIELD() are strings, all arguments are compared
as strings. If all arguments are numbers, they are compared as
numbers.  Otherwise, the arguments are compared as double.

If str is NULL, the return value is 0 because
NULL fails equality comparison with any value.
FIELD() is the complement of ELT().
