Your task is to fill out the solve(lines) method of the sample
starter-code (defining extra functions if you need them).  Your program
should take, as input, the entire sample input displayed below, and
produce the entire sample output, as displayed below.  You should save the
sample input as a file like "a4.in", and the sample code as a file like
"a4.py", and then invoke your program to test it by entering the command
"python a4.py < a4.in", which will feed the sample input, as input to your
program.  A good place to start is to add "print lines" to solve() and see
what it looks like.

In the input, "A < B" means that A is in front of B, and "A > B" means
that A is behind B.  The input is divided up into several cases.  Cases
are separated by at least one blank line.  (The sample code already takes
care of input for you, but you still need to understand how the input is
organized.)

In the output for each input case, you should write whichever ONE of the
following statements is true of the input case.  "The only possible order
is [...]", "One possible order is [...]", or "The input is
self-contradictory", where "[...]" is a list of names, separated by spaces
(no commas, no other symbols), as in the sample output below.

Sample input:
------------------------------------------------
Alice > Bob
Bob < Cho
Cho > Dai
Dai > Alice

Alice < Bob
Cho < Bob
Cho < Dai
Dai < Alice

Alice < Bob
Cho > Bob
Cho < Dai
Dai < Alice

A < B
C < D

1 < 17
5 > 3
200 > 5
16 > 5
17 > 16
200 > 17
1 < 3
------------------------------------------------

Sample output:
------------------------------------------------
The only possible order is Bob Alice Dai Cho
The only possible order is Cho Dai Alice Bob
The input is self contradictory
One possible order is C D A B
The only possible order is 1 3 5 16 17 200
------------------------------------------------

Sample starter-code:
------------------------------------------------
import sys

def solve(lines):
   # if lines is empty, there is no case to solve, so just return nil
   if len(lines) == 0: return

   # otherwise, we need to solve the case, here.

def main():
   lines = sys.stdin.read().split('\n')
   lastblank = -1
   for i in range(len(lines)+1):
       # if we read a blank line or reach the end of the input,
       # then we may have finished reading a test case (so we solve it)
       if i == len(lines) or len(lines[i]) == 0:
           solve(lines[lastblank+1:i])
           lastblank = i

main()