first-order arithmetic decision procedure?



I am working on a program analysis tool for a research project as part
of my graduate studies, and I need a decision procedure for first
order arithmetic.  I need to be able to decide whether expressions
like:

EXISTS x:  x > 0  AND  FORALL a:  x*a >= 0

are true.

Tarski's theorem says that when the variables range over real numbers,
quantifier elimination implies the decidability of such expressions.
Ideally I would also like to get values for existentially quantified
variables if the expression is true (e.g., x = 1, although the example
is false), but this is not essential.

I would like to use Maxima because it is open source and free, but I
have not been able to figure out whether Maxima includes the necessary
expressive power to solve such expressions.  Can anyone tell me
either: that this is not possible, or, if it is possible, how to do
it?  Thanks.

--Gary