decomposing large packages

[mario.blunk.gplus@gmail.com]

Hello,
I have a package P (spec and body) which is becoming greater and greater as more functionality is implemented. Is there a way to decompose the package into smaller packages S1, S2, etc ? I'm thinking of a kind of "include" directive that refers to the files where stuff has been moved. Example:

The package P in the current state contains:

package P is
 type thing is ...
 procedure m (value : in thing);
 procedure n (value : in thing; temp : in natural);
end package P;

package body P is:
 procedure m (value : in thing) is ...
 procedure n (value : in thing; temp : in natural) is ...
end package body P;

From inside package P the procedure n can be called which is fine so far.
Now I want to take out procedure n and move it into package S1:

package S1 is
 procedure n (value : in thing; temp : in natural);
end package S1;

package body S1 is:
 procedure n (value : in thing; temp : in natural) is ...
end package body S1;

Now procedure n must be visible from inside package P. How can I accomplish that ? I played with child units but did not get what I wanted. The parent P unit must see the procedures and functions of the child units P.S1, P.S2, ... but the compiler complains that procedure n is not defined for type thing in P.
Thanks for your help in advance.