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Circular assignments

A circularity error occurs if there is a cycle of zero-delay assignments amongst the union of all assignments in all layers. Thus for example, the following program:

        layer Q : {
          x := y;
        }
        layer P : {
          y := x;
          next(x) := y;
        }
        P refines Q;

is erroneous, even though it is functionally equivalent to the non-circular program:

        y := x;
        next(x) := y;