test Invariant
    act:    # create 2 named values
        period: Period();
        today: today()
    assert: # check the values
        period.Start = today;
        not period.Days;
        period.End = today
    act:    # make a change
        period.set(Days: 5)
    assert: # validate
        period.End = today + 5
    act:    # make another change
        period.set(Start: today + 7)
    assert: # validate
        period.Start = today + 7;
        period.End = today + 12
    act:    # make one more change
        period.set(End: today - 3 * Days)
    assert: # validate
        period.Start = today - 20;
        period.End = today - 15
    act:    # Days < 0 violates invariant
        period.set(Days: -3)
    assert: # validate that it's ignored
        period.Days = 5;
        period.Start = today - 20;
        period.End = today - 15