German,

G> Shortcut evaluation. Consider you have:

if (a>>b) and (c<d) then....

G> So, if b is not less than a then it is not necessary to check c < d.
G> Therefore, you immediately return false.

if I recall well, Firebird 1 does full boolean evaluation whereas
Firebird 1.5 does shortcut evaluation. I'm not 100% sure, though.
I think you'll find the final answer by researching the archives for
Mike Nordell's posts.
Ciao
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