In general, there are many ways to express the same linear relationship
between variables. clp(Q,R) does not care to distinguish between
them, but the user might. The predicate
ordering(+Spec) gives you some control over the
variable ordering. Suppose that instead of B, you want
Mp to be the defined variable:
clp(r) ?- mg(P,12,0.01,B,Mp).
{B=1.1268250301319698*P-12.682503013196973*Mp}
This is achieved with:
clp(r) ?- mg(P,12,0.01,B,Mp), ordering([Mp]).
{Mp= -0.0788487886783417*B+0.08884878867834171*P}
One could go one step further and require P to appear before
(to the left of) B in a addition:
clp(r) ?- mg(P,12,0.01,B,Mp), ordering([Mp,P]).
{Mp=0.08884878867834171*P-0.0788487886783417*B}
Spec in ordering(+Spec) is either a list of
variables with the intended ordering, or of the form
A<B. The latter form means that A goes to the
left of B. In fact, ordering([A,B,C,D]) is shorthand for:
ordering(A < B), ordering(A < C), ordering(A < D),
ordering(B < C), ordering(B < D),
ordering(C < D)
The ordering specification only affects the final presentation of the
constraints. For all other operations of clp(Q,R), the ordering is immaterial.
Note that ordering/1 acts like a constraint: you can put it anywhere
in the computation, and you can submit multiple specifications.
clp(r) ?- ordering(B < Mp), mg(P,12,0.01,B,Mp).
{B= -12.682503013196973*Mp+1.1268250301319698*P}
clp(r) ?- ordering(B < Mp), mg(P,12,0.01,B,Mp), ordering(P < Mp).
{P=0.8874492252651537*B+11.255077473484631*Mp}