Fixed last unit test of book 1 chapter 10.

tests/test_b1_ch10.py

@@ -45,11 +45,17 @@ class TestB1Ch10(unittest.TestCase):
                 )
             ),
             ('(7x2y3)^2/(7x2y3)',
-                N('*',
+                N('/',
+                  N('^',
+                    N('*',
+                      N('*', L(7), N('^', L('x'), L(2))),
+                      N('^', L('y'), L(3))
+                    ),
+                    L(2)),
                   N('*',
-                    N('-', N('^', L('x'), L(3))),
-                    N('-', L(2))),
-                  N('^', L('x'), L(5))
+                    N('*', L(7), N('^', L('x'), L(2))),
+                    N('^', L('y'), L(3))
+                  )
                 )
             ),
             ])