Added helper function to check if a node matches a fraction format.

src/rules/utils.py

+from ..node import ExpressionLeaf as L, OP_MUL, OP_DIV
+
+
 def gcd(a, b):
     """
     Return greatest common divisor using Euclid's Algorithm.
@@ -20,3 +23,34 @@ def least_common_multiple(*args):
     Return lcm of args.
     """
     return reduce(lcm, args)
+
+
+def is_fraction(node, nominator, denominator):
+    """
+    Check if a node represents the fraction of a given nominator and
+    denominator.
+
+    >>> from ..node import ExpressionLeaf as L
+    >>> l1, l2, a = L('a'), L(1), L(2)
+    >>> is_fraction(a / l2, a, 2)
+    True
+    >>> is_fraction(l1 / l2 * a, a, 2)
+    True
+    >>> is_fraction(l2 / l1 * a, a, 2)
+    False
+    """
+    if node.is_op(OP_DIV):
+        nom, denom = node
+
+        return nom == nominator and denom == denominator
+
+    if node.is_op(OP_MUL):
+        # 1 / denominator * nominator
+        # nominator * 1 / denominator
+        left, right = node
+        fraction = L(1) / denominator
+
+        return (left == nominator and right == fraction) \
+               or (right == nominator and left == fraction)
+
+    return False

tests/test_rules_utils.py

 import unittest
 
-from src.rules.utils import least_common_multiple
+from src.rules.utils import least_common_multiple, is_fraction
+from tests.rulestestcase import tree
 
 
 class TestRulesUtils(unittest.TestCase):
@@ -9,3 +10,11 @@ class TestRulesUtils(unittest.TestCase):
         self.assertEqual(least_common_multiple(5, 6), 30)
         self.assertEqual(least_common_multiple(5, 6, 15), 30)
         self.assertEqual(least_common_multiple(2, 4), 4)
+
+    def test_is_fraction(self):
+        l1, a = tree('1, a')
+
+        self.assertTrue(is_fraction(a / 2, a, 2))
+        self.assertTrue(is_fraction(l1 / 2 * a, a, 2))
+        self.assertTrue(is_fraction(a * (l1 / 2), a, 2))
+        self.assertFalse(is_fraction(l1 / 3 * a, a, 2))