diff --git a/tests/test_core.py b/tests/test_core.py
index 680ce6e..5249b0b 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py
@@ -1,7 +1,8 @@
 from types import MappingProxyType
+from collections import OrderedDict
 
 from unification import var
-from unification.core import reify, unify
+from unification.core import reify, unify, unground_lvars
 
 
 def test_reify():
@@ -15,11 +16,19 @@ def test_reify():
     assert reify(z, MappingProxyType(s)) == (1, 2)
 
 
-def test_reify_dict():
+def test_reify_Mapping():
     x, y = var(), var()
     s = {x: 2, y: 4}
-    e = {1: x, 3: {5: y}}
-    assert reify(e, s) == {1: 2, 3: {5: 4}}
+    e = [(1, x), (3, {5: y})]
+    expected_res = [(1, 2), (3, {5: 4})]
+    assert reify(dict(e), s) == dict(expected_res)
+    assert reify(OrderedDict(e), s) == OrderedDict(expected_res)
+
+
+def test_reify_Set():
+    x, y = var(), var()
+    assert reify({1, 2, x, y}, {x: 3}) == {1, 2, 3, y}
+    assert reify(frozenset({1, 2, x, y}), {x: 3}) == frozenset({1, 2, 3, y})
 
 
 def test_reify_list():
@@ -37,6 +46,19 @@ def test_reify_complex():
     assert reify(e, s) == {1: [2], 3: (4, 5)}
 
 
+def test_unify_slice():
+    x = var("x")
+    y = var("y")
+
+    assert unify(slice(1), slice(1), {}) == {}
+    assert unify(slice(1, 2, 3), x, {}) == {x: slice(1, 2, 3)}
+    assert unify(slice(1, 2, None), slice(x, y), {}) == {x: 1, y: 2}
+
+
+def test_reify_slice():
+    assert reify(slice(1, var(2), 3), {var(2): 10}) == slice(1, 10, 3)
+
+
 def test_unify():
     assert unify(1, 1, {}) == {}
     assert unify(1, 2, {}) is False
@@ -59,10 +81,11 @@ def test_unify_seq():
 
 
 def test_unify_set():
-    x = var("x")
-    assert unify(set((1, 2)), set((1, 2)), {}) == {}
-    assert unify(set((1, x)), set((1, 2)), {}) == {x: 2}
-    assert unify(set((x, 2)), set((1, 2)), {}) == {x: 1}
+    x, y = var(), var()
+    assert unify({1, 2}, {1, 2}, {}) == {}
+    assert unify({1, x}, {1, 2}, {}) == {x: 2}
+    assert unify({x, 2}, {1, 2}, {}) == {x: 1}
+    assert unify({1, y, x}, {2, 1}, {x: 2}) is False
 
 
 def test_unify_dict():
@@ -80,3 +103,15 @@ def test_unify_complex():
 
     assert unify({1: (2, 3)}, {1: (2, var(5))}, {}) == {var(5): 3}
     assert unify({1: [2, 3]}, {1: [2, var(5)]}, {}) == {var(5): 3}
+
+
+def test_unground_lvars():
+    assert unground_lvars((1, 2), {}) == set()
+    assert unground_lvars((1, [var("a"), [var("b"), 2], 3]), {}) == {var("a"), var("b")}
+    assert unground_lvars((1, [var("a"), [var("b"), 2], 3]), {var("a"): 4}) == {
+        var("b")
+    }
+    assert (
+        unground_lvars((1, [var("a"), [var("b"), 2], 3]), {var("a"): 4, var("b"): 5})
+        == set()
+    )
diff --git a/tests/test_more.py b/tests/test_more.py
index ce676e3..cd1b808 100644
--- a/tests/test_more.py
+++ b/tests/test_more.py
@@ -61,20 +61,6 @@ def test_objects_full():
     ) == Foo(1, Bar(Foo(2, 3)))
 
 
-def test_unify_slice():
-    x = var("x")
-    y = var("y")
-
-    assert unify(slice(1), slice(1), {}) == {}
-    assert unify(slice(1, 2, 3), x, {}) == {x: slice(1, 2, 3)}
-    assert unify(slice(1, 2, None), slice(x, y), {}) == {x: 1, y: 2}
-
-
-def test_reify_slice():
-    x = var("x")
-    assert reify(slice(1, var(2), 3), {var(2): 10}) == slice(1, 10, 3)
-
-
 @unifiable
 class A(object):
     def __init__(self, a, b):
@@ -105,7 +91,7 @@ def __eq__(self, other):
         return self.__dict__ == other.__dict__
 
 
-def test_unifiable():
+def test_unifiable_slots():
     x = var("x")
     f = Aslot(1, 2)
     g = Aslot(1, x)
diff --git a/unification/core.py b/unification/core.py
index ce8a7b8..2bb2916 100644
--- a/unification/core.py
+++ b/unification/core.py
@@ -1,37 +1,47 @@
 from toolz import assoc
 from operator import length_hint
-from collections.abc import Iterator, Mapping
+from functools import partial
+from collections import OrderedDict
+from collections.abc import Iterator, Mapping, Set
 
 from .utils import transitive_get as walk
 from .variable import isvar
 from .dispatch import dispatch
 
 
-@dispatch(Iterator, Mapping)
-def _reify(t, s):
-    return iter(reify(arg, s) for arg in t)
+@dispatch(object, Mapping)
+def _reify(o, s):
+    return o
 
 
-@dispatch(tuple, Mapping)
-def _reify(t, s):
-    return tuple(reify(iter(t), s))
+def _reify_Iterable(type_ctor, t, s):
+    return type_ctor(reify(a, s) for a in t)
 
 
-@dispatch(list, Mapping)
-def _reify(t, s):
-    return list(reify(iter(t), s))
+for seq, ctor in (
+    (tuple, tuple),
+    (list, list),
+    (Iterator, iter),
+    (set, set),
+    (frozenset, frozenset),
+):
+    _reify.add((seq, Mapping), partial(_reify_Iterable, ctor))
 
 
-@dispatch(dict, Mapping)
-def _reify(d, s):
-    return dict((k, reify(v, s)) for k, v in d.items())
+def _reify_Mapping(ctor, d, s):
+    return ctor((k, reify(v, s)) for k, v in d.items())
 
 
-@dispatch(object, Mapping)
-def _reify(o, s):
-    return o  # catch all, just return the object
+for seq in (dict, OrderedDict):
+    _reify.add((seq, Mapping), partial(_reify_Mapping, seq))
+
 
+@_reify.register(slice, Mapping)
+def _reify_slice(o, s):
+    return slice(*reify((o.start, o.stop, o.step), s))
 
+
+@dispatch(object, Mapping)
 def reify(e, s):
     """Replace variables of an expression with their substitutions.
 
@@ -46,16 +56,16 @@ def reify(e, s):
     {1: 2, 3: (4, 5)}
     """
     if isvar(e):
-        return reify(s[e], s) if e in s else e
+        e = walk(e, s)
     return _reify(e, s)
 
 
 @dispatch(object, object, Mapping)
 def _unify(u, v, s):
-    return False  # catch all
+    return False
 
 
-def _unify_seq(u, v, s):
+def _unify_Sequence(u, v, s):
     len_u = length_hint(u, -1)
     len_v = length_hint(v, -1)
 
@@ -70,19 +80,19 @@ def _unify_seq(u, v, s):
 
 
 for seq in (tuple, list, Iterator):
-    _unify.add((seq, seq, Mapping), _unify_seq)
+    _unify.add((seq, seq, Mapping), _unify_Sequence)
 
 
-@dispatch((set, frozenset), (set, frozenset), Mapping)
-def _unify(u, v, s):
+@_unify.register(Set, Set, Mapping)
+def _unify_Set(u, v, s):
     i = u & v
     u = u - i
     v = v - i
-    return _unify(sorted(u), sorted(v), s)
+    return _unify(iter(u), iter(v), s)
 
 
-@dispatch(dict, dict, Mapping)
-def _unify(u, v, s):
+@_unify.register(Mapping, Mapping, Mapping)
+def _unify_Mapping(u, v, s):
     if len(u) != len(v):
         return False
     for key, uval in u.items():
@@ -94,6 +104,11 @@ def _unify(u, v, s):
     return s
 
 
+@_unify.register(slice, slice, dict)
+def _unify_slice(u, v, s):
+    return unify((u.start, u.stop, u.step), (v.start, v.stop, v.step), s)
+
+
 @dispatch(object, object, Mapping)
 def unify(u, v, s):
     """Find substitution so that u == v while satisfying s.
@@ -113,6 +128,28 @@ def unify(u, v, s):
     return _unify(u, v, s)
 
 
-@dispatch(object, object)
-def unify(u, v):
+@unify.register(object, object)
+def unify_NoMap(u, v):
     return unify(u, v, {})
+
+
+def unground_lvars(u, s):
+    """Return the unground logic variables from a term and state."""
+
+    lvars = set()
+    _reify_object = _reify.dispatch(object, Mapping)
+
+    def _reify_var(u, s):
+        nonlocal lvars
+
+        if isvar(u):
+            lvars.add(u)
+        return u
+
+    _reify.add((object, Mapping), _reify_var)
+    try:
+        reify(u, s)
+    finally:
+        _reify.add((object, Mapping), _reify_object)
+
+    return lvars
diff --git a/unification/more.py b/unification/more.py
index a2acee2..916e7f7 100644
--- a/unification/more.py
+++ b/unification/more.py
@@ -1,5 +1,6 @@
-from .core import unify, reify
-from .dispatch import dispatch
+from collections.abc import Mapping
+
+from .core import unify, reify, _unify, _reify
 
 
 def unifiable(cls):
@@ -24,8 +25,8 @@ def unifiable(cls):
     >>> unify(a, b, {})
     {~x: 2}
     """
-    _unify.add((cls, cls, dict), unify_object)
-    _reify.add((cls, dict), reify_object)
+    _unify.add((cls, cls, Mapping), unify_object)
+    _reify.add((cls, Mapping), reify_object)
 
     return cls
 
@@ -74,12 +75,6 @@ def _reify_object_slots(o, s):
         return newobj
 
 
-@dispatch(slice, dict)
-def _reify(o, s):
-    """Reify a Python ``slice`` object."""
-    return slice(*reify((o.start, o.stop, o.step), s))
-
-
 def unify_object(u, v, s):
     """Unify two Python objects.
 
@@ -108,9 +103,3 @@ def unify_object(u, v, s):
         )
     else:
         return unify(u.__dict__, v.__dict__, s)
-
-
-@dispatch(slice, slice, dict)
-def _unify(u, v, s):
-    """Unify a Python ``slice`` object."""
-    return unify((u.start, u.stop, u.step), (v.start, v.stop, v.step), s)
diff --git a/unification/utils.py b/unification/utils.py
index 317a820..6f19930 100644
--- a/unification/utils.py
+++ b/unification/utils.py
@@ -73,14 +73,6 @@ def reverse_dict(d):
     return result
 
 
-def xfail(func):
-    try:
-        func()
-        raise Exception("XFailed test passed")  # pragma:nocover
-    except Exception:
-        pass
-
-
 def freeze(d):
     """Freeze container to hashable a form.