SPARK - division by zero problem

Ada Programming

While playing with SPARK (I also agree that comp.lang.ada is adequate for SPARK content) I have found a problem that can be shown in the following stripped-down example:

package My_Package is

    procedure Divide (X : in Integer; Y : in Integer; Z : out Integer);
    --# derives Z from X, Y;
    --# pre Y /= 0;
    --# post Z = X / Y;

end My_Package;

package body My_Package is

    procedure Divide (X : in Integer; Y : in Integer; Z : out Integer) is
        Z := X / Y;  -- this is line 5
    end Divide;

end My_Package;

In this example I would expect the VC associated with rtc on line 5 above to be discharged based on the hypothesis from Divide's precondition: that is, Y is known to be non-zero, so division by zero in line 5 cannot happen.

Tools are invoked in the following order:

$ sparkmake
$ spark -index_file=spark.idx -vcg -config_file=config.cfg -output_dir=spark my_package.adb
$ sparksimp
$ pogs

The summary says that the VC in question in undischarged.
Is there anything missing?

The tools come from Ubuntu packages and announce themselves as GPL 2011.                                            
[email protected] says...

The undischarged VC I get is:
H1:    y <> 0 .
H2:    x >= - 2147483648 .
H3:    x <= 2147483647 .
H4:    y >= - 2147483648 .
H5:    y <= 2147483647 .
H6:    integer__size >= 0 .
C1:    x div y <= 2147483647 .

which is false for x = -2147483648, y = -1


Right, I fell into the trap by focusing on Y /= 0 condition. Indeed, the assymetry of Integer'Range is causing troubles in this case.
Thank you for response.