Serializable Histories
A history is serializable if it is equivalent to a serial history
For example, H1 = r1[x] r2[x] w1[x] c1 w2[y] c2is equivalent to H4 = r2[x] w2[y] c2 r1[x] w1[x] c1(r2[x] and w1[x] are in the same order in H1 and H4.)
Therefore, H1 is serializable.