If your Prolog-in-CL uses slot readers/accessors, dependencies will arise. So the position (or whatever GUI layout attribute you have in mind) can be calculated by a cl-prolog operating over CLOS instances via CLOS slot accessor GFs, and the necessary dataflow will arise naturally.
The invocation of the cl-prolog computation will have to require dynamic insertion of initializing/configuring facts. The cell rule calculating the result will specify these facts from other cell values, and supply the calculating form. This then neatly embeds Prolog within Cells, with the Prolog both driving and driven by Cells. Yet one gets (I presume) the greater performance from a dedicated prolog working on a fixed set of facts (for the duration of a single calculation).
Not very sure of the above, btw.
kenny