Hello,
I am trying to learn about constraints from Foundation of constraint
satisfication
book by tsang whose chapters are available online .

In chapter , the algorithm for PC-1 (path consistency) following
notion is used:

Y^n <--- C ;

Y^k (i,j) <-- Y^k-1 (i,j) AND Y^k-1 (i,k) * Y^k-1 (k,k) * Y^k-1
(k,j) ;

where n is number of total variables , C is a set of binary
constraints, and
k is the k-th variable .

Now lets say Z={A,B,C} ==> variables
C={Cab , Cbc , Cca} be binary constraints

My question is :
What does Y^1 or Y^A represent actually ?
Is it a set like this Y^A = { Caa , Cab , Cac}

I the book it say Y^k (i,j) represents a constraint Ci,j in the set
Y^k .

Thanks