How to use the pyperformance.benchmarks.bm_deltablue.StayConstraint function in pyperformance

time is measured to change a variable on either side of the
    mapping and to change the scale and offset factors.
    """
    global planner
    planner = Planner()
    scale = Variable("scale", 10)
    offset = Variable("offset", 1000)
    src = None

    dests = OrderedCollection()

    for i in range(n):
        src = Variable("src%s" % i, i)
        dst = Variable("dst%s" % i, i)
        dests.append(dst)
        StayConstraint(src, Strength.NORMAL)
        ScaleConstraint(src, scale, offset, dst, Strength.REQUIRED)

    change(src, 17)

    if dst.value != 1170:
        print("Projection 1 failed")

    change(dst, 1050)

    if src.value != 5:
        print("Projection 2 failed")

    change(scale, 5)

    for i in range(n - 1):
        if dests[i].value != (i * 5 + 1000):

def __init__(self, v, string):
        super(StayConstraint, self).__init__(v, string)

for i in range(n + 1):
        name = "v%s" % i
        v = Variable(name)

        if prev is not None:
            EqualityConstraint(prev, v, Strength.REQUIRED)

        if i == 0:
            first = v

        if i == n:
            last = v

        prev = v

    StayConstraint(last, Strength.STRONG_DEFAULT)
    edit = EditConstraint(first, Strength.PREFERRED)
    edits = OrderedCollection()
    edits.append(edit)
    plan = planner.extract_plan_from_constraints(edits)

    for i in range(100):
        first.value = i
        plan.execute()

        if last.value != i:
            print("Chain test failed.")