ORA-FO-DATETIME-SIGNATURES Section 9.7.1, 9.7.2, 9.7.3, 9.7.4

*Functions and Operators, Section 9.7.1, 9.7.2, 9.7.3, 9.7.4*

The function signatures need "?" after argument types, since $srcval1, 
$srcval2, etc. can be the empty sequence.
And the return also needs a "?", since "If either argument is the empty 
sequence, returns the empty sequence".

See also

http://lists.w3.org/Archives/Public/public-qt-comments/2003Jun/0217.html