===== Testing subtract =====
2/3 - 1/6 = 1/2
1/5 - 4/9 = -11/45
3/7 - 12/21 = -1/7
5/8 - 3/16 = 7/16
7/8 - 3/12 = 5/8
0/8 - 3/16 = -3/16
1/1 - 3/16 = 13/16
5/8 - -3/16 = 13/16
1/5 - -4/9 = 29/45
-1/5 - -4/9 = 11/45
5/5 - 5/5 = 0/1