Question: Why does 'is' command FAIL here?

I tried to find out whether the expression



is positive under the given assumptions:

is(signum(g1)=1) assuming mu>0, sigma2>0, phi>0, sigma2<>mu;


Though I am able to verify this boolean test by using 'explore', the 'is' command yields FAIL and is obviously not able to determine whether g1 is positive under the given assumptions.

Why does 'is' FAIL here and what can I do to have Maple recognize the truth of such boolean tests?

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