Empty sequences in "for" clause bindings

Dear all,
 
how “for” clause tuples are generated when some of the bounded variables match the empty sequence ?
 
For example should the following query return (1, 2, 3), an error or anything else ?
 
for $a in (), $b in (1, 2, 3)
return ($a, $b)
 
 
Regards
 
 
Stéphane
Mbaye
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GAEL Consultant 
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