Identify the fault.
左边程序的fault是循环条件，不会遍历到x[0]
右边程序的fault是程序本身是寻找最前面的0，而目的是寻找最后面的0
If possible, identify a test case that does not execute the fault. (Reachability)
左边: x=[] y=4
右边: x=[]
If possible, identify a test case that executes the fault, but does not result in an error state.
左边: x=[2,3,4] y=4
右边: x=[1,0,2]
If possible identify a test case that results in an error, but not a failure.
左边: x=[2,3,4] y=5
右边: x=[1,2]