Welcome to SWI-Prolog (threaded, 64 bits, version 9.2.9)
?- use_module(library(clpBNR)).
% *** clpBNR v0.12.2 ***.
true.
?- {TotalOwed == TotalTax - TotalPayments}.
TotalOwed::real(-1.0Inf, 1.0Inf),
TotalTax::real(-1.0Inf, 1.0Inf),
TotalPayments::real(-1.0Inf, 1.0Inf).
?- {TotalOwed == TotalTax - TotalPayments}, TotalTax = 10, TotalPayments = 5.
TotalOwed = TotalPayments, TotalPayments = 5,
TotalTax = 10.
If you restrict yourself to the pure subset of prolog, you can even express complicated computation involving conditions or recusions.
However, this means that your graph is now encoded into the prolog code itself, which is harder to manipulate, but still fully manipulable in prolog itself.
To understand why OMR is so neglected is because most people widely underestimate the difficulty of the task. It has a specific blend of the most extreme shapes combined with an extremely complicated graphical grammar...