diff --git a/tests/nocover/test_example_quality.py b/tests/nocover/test_example_quality.py
@@ -22,7 +22,11 @@
 import operator
 from decimal import Decimal
 from fractions import Fraction
-from collections import Counter, OrderedDict
+try:
+    from collections import Counter, OrderedDict
+except ImportError:
+    from counter import Counter
+    from ordereddict import OrderedDict
 
 import pytest
 from hypothesis import Settings, assume
@@ -107,7 +111,7 @@ def test_minimize_one_of():
 
 def test_minimize_mixed_list():
     mixed = minimal(lists(integers() | text()), lambda x: len(x) >= 10)
-    assert set(mixed).issubset({0, ''})
+    assert set(mixed).issubset(set((0, '')))
 
 
 def test_minimize_longer_string():
@@ -120,15 +124,15 @@ def test_minimize_longer_list_of_strings():
 
 def test_minimize_3_set():
     assert minimal(sets(integers()), lambda x: len(x) >= 3) in (
-        {0, 1, 2},
-        {-1, 0, 1},
+        set((0, 1, 2)),
+        set((-1, 0, 1)),
     )
 
 
 def test_minimize_3_set_of_tuples():
     assert minimal(
         sets(tuples(integers())),
-        lambda x: len(x) >= 2) == {(0,), (1,)}
+        lambda x: len(x) >= 2) == set(((0,), (1,)))
 
 
 def test_minimize_sets_of_sets():
@@ -251,7 +255,7 @@ def test_dictionary(dict_class):
         dictionaries(keys=integers(), values=text(), dict_class=dict_class),
         lambda t: len(t) >= 3)
     assert isinstance(x, dict_class)
-    assert set(x.values()) == {''}
+    assert set(x.values()) == set(('',))
     for k in x:
         if k < 0:
             assert k + 1 in x

diff --git a/tests/nocover/test_statistical_distribution.py b/tests/nocover/test_statistical_distribution.py
@@ -30,7 +30,10 @@
 import re
 import math
 import random
-from collections import Counter
+try:
+    from collections import Counter
+except ImportError:
+    from counter import Counter
 
 import pytest
 import hypothesis.internal.reflection as reflection
@@ -49,6 +52,31 @@
 MAX_RUNS = MIN_RUNS * 20
 
 
+if not hasattr(math, 'erf'):
+    def erf(x):
+        # constants
+        a1 =  0.254829592
+        a2 = -0.284496736
+        a3 =  1.421413741
+        a4 = -1.453152027
+        a5 =  1.061405429
+        p  =  0.3275911
+
+        # Save the sign of x
+        sign = 1
+        if x < 0:
+            sign = -1
+        x = abs(x)
+
+        # A & S 7.1.26
+        t = 1.0/(1.0 + p*x)
+        y = 1.0 - (((((a5*t + a4)*t) + a3)*t + a2)*t + a1)*t*math.exp(-x*x)
+
+        return sign*y
+
+    math.erf = erf
+
+
 def cumulative_normal(x):
     return 0.5 * (1 + math.erf(x / math.sqrt(2)))