diff --git a/docs/new_sets_doc.md b/docs/new_sets_doc.md
index 4f7fde22..18b6507d 100755
--- a/docs/new_sets_doc.md
+++ b/docs/new_sets_doc.md
@@ -246,6 +246,29 @@ Returns the union of several sets.
 The set union of all sets in `*args`.
 
 
+<a id="sets.mutable_union"></a>
+
+## sets.mutable_union
+
+<pre>
+sets.mutable_union(<a href="#sets.mutable_union-a">a</a>, <a href="#sets.mutable_union-b">b</a>)
+</pre>
+
+Modify set `a` adding elements from `b` to it.
+
+**PARAMETERS**
+
+
+| Name  | Description | Default Value |
+| :------------- | :------------- | :------------- |
+| <a id="sets.mutable_union-a"></a>a |  A set, as returned by <code>sets.make()</code>.   |  none |
+| <a id="sets.mutable_union-b"></a>b |  A set, as returned by <code>sets.make()</code>.   |  none |
+
+**RETURNS**
+
+The set `a` with all elements appearing in `b` added to it.
+
+
 <a id="sets.difference"></a>
 
 ## sets.difference
@@ -269,6 +292,29 @@ Returns the elements in `a` that are not in `b`.
 A set containing the elements that are in `a` but not in `b`.
 
 
+<a id="sets.mutable_difference"></a>
+
+## sets.mutable_difference
+
+<pre>
+sets.mutable_difference(<a href="#sets.mutable_difference-a">a</a>, <a href="#sets.mutable_difference-b">b</a>)
+</pre>
+
+Modify set `a` removing elements from `b` from it.
+
+**PARAMETERS**
+
+
+| Name  | Description | Default Value |
+| :------------- | :------------- | :------------- |
+| <a id="sets.mutable_difference-a"></a>a |  A set, as returned by <code>sets.make()</code>.   |  none |
+| <a id="sets.mutable_difference-b"></a>b |  A set, as returned by <code>sets.make()</code>.   |  none |
+
+**RETURNS**
+
+The set `a` with all elements appearing in `b` removed from it.
+
+
 <a id="sets.length"></a>
 
 ## sets.length
diff --git a/lib/new_sets.bzl b/lib/new_sets.bzl
index cd90a30e..02cf2cca 100644
--- a/lib/new_sets.bzl
+++ b/lib/new_sets.bzl
@@ -189,6 +189,19 @@ def _union(*args):
     """
     return struct(_values = dicts.add(*[s._values for s in args]))
 
+def _mutable_union(a, b):
+    """Modify set `a` adding elements from `b` to it.
+
+    Args:
+      a: A set, as returned by `sets.make()`.
+      b: A set, as returned by `sets.make()`.
+
+    Returns:
+      The set `a` with all elements appearing in `b` added to it.
+    """
+    a._values.update(b._values)
+    return a
+
 def _difference(a, b):
     """Returns the elements in `a` that are not in `b`.
 
@@ -201,6 +214,20 @@ def _difference(a, b):
     """
     return struct(_values = {e: None for e in a._values.keys() if e not in b._values})
 
+def _mutable_difference(a, b):
+    """Modify set `a` removing elements from `b` from it.
+
+    Args:
+      a: A set, as returned by `sets.make()`.
+      b: A set, as returned by `sets.make()`.
+
+    Returns:
+      The set `a` with all elements appearing in `b` removed from it.
+    """
+    for item in b._values.keys():
+        a._values.pop(item)
+    return a
+
 def _length(s):
     """Returns the number of elements in a set.
 
@@ -234,7 +261,9 @@ sets = struct(
     disjoint = _disjoint,
     intersection = _intersection,
     union = _union,
+    mutable_union = _mutable_union,
     difference = _difference,
+    mutable_difference = _mutable_difference,
     length = _length,
     remove = _remove,
     repr = _repr,
diff --git a/tests/new_sets_tests.bzl b/tests/new_sets_tests.bzl
index e73b7d46..9e7faa38 100644
--- a/tests/new_sets_tests.bzl
+++ b/tests/new_sets_tests.bzl
@@ -98,17 +98,29 @@ def _union_test(ctx):
     """Unit tests for sets.union."""
     env = unittest.begin(ctx)
 
-    asserts.new_set_equals(env, sets.make(), sets.union())
-    asserts.new_set_equals(env, sets.make([1]), sets.union(sets.make([1])))
-    asserts.new_set_equals(env, sets.make(), sets.union(sets.make(), sets.make()))
-    asserts.new_set_equals(env, sets.make([1]), sets.union(sets.make(), sets.make([1])))
-    asserts.new_set_equals(env, sets.make([1]), sets.union(sets.make([1]), sets.make()))
-    asserts.new_set_equals(env, sets.make([1]), sets.union(sets.make([1]), sets.make([1])))
-    asserts.new_set_equals(env, sets.make([1, 2]), sets.union(sets.make([1]), sets.make([1, 2])))
-    asserts.new_set_equals(env, sets.make([1, 2]), sets.union(sets.make([1]), sets.make([2])))
+    s = sets.make()
+    s = sets.mutable_union(s, sets.make())
+    asserts.new_set_equals(env, sets.make(), s)
+    s = sets.make()
+    s = sets.mutable_union(s, sets.make([1]))
+    asserts.new_set_equals(env, sets.make([1]), s)
+    s = sets.make([1])
+    s = sets.mutable_union(s, sets.make())
+    asserts.new_set_equals(env, sets.make([1]), s)
+    s = sets.make([1])
+    s = sets.mutable_union(s, sets.make([1]))
+    asserts.new_set_equals(env, sets.make([1]), s)
+    s = sets.make([1])
+    s = sets.mutable_union(s, sets.make([1, 2]))
+    asserts.new_set_equals(env, sets.make([1, 2]), s)
+    s = sets.make([1])
+    s = sets.mutable_union(s, sets.make([2]))
+    asserts.new_set_equals(env, sets.make([1, 2]), s)
 
     # If passing a list, verify that duplicate elements are ignored.
-    asserts.new_set_equals(env, sets.make([1, 2]), sets.union(sets.make([1, 1]), sets.make([1, 2])))
+    s = sets.make([1, 1])
+    s = sets.mutable_union(s, sets.make([1, 2]))
+    asserts.new_set_equals(env, sets.make([1, 2]), s)
 
     return unittest.end(env)
 
@@ -132,6 +144,38 @@ def _difference_test(ctx):
 
 difference_test = unittest.make(_difference_test)
 
+def _mutable_difference_test(ctx):
+    """Unit tests for sets.difference."""
+    env = unittest.begin(ctx)
+
+    s = sets.make()
+    s = sets.mutable_difference(s, sets.make())
+    asserts.new_set_equals(env, sets.make(), s)
+    s = sets.make()
+    s = sets.mutable_difference(s, sets.make([1]))
+    asserts.new_set_equals(env, sets.make(), s)
+    s = sets.make([1])
+    s = sets.mutable_difference(s, sets.make())
+    asserts.new_set_equals(env, sets.make([1]), s)
+    s = sets.make([1])
+    s = sets.mutable_difference(s, sets.make([1]))
+    asserts.new_set_equals(env, sets.make(), s)
+    s = sets.make([1])
+    s = sets.mutable_difference(s, sets.make([1, 2]))
+    asserts.new_set_equals(env, sets.make(), s)
+    s = sets.make([1])
+    s = sets.mutable_difference(s, sets.make([2]))
+    asserts.new_set_equals(env, sets.make([1]), s)
+
+    # If passing a list, verify that duplicate elements are ignored.
+    s = sets.make([1, 2])
+    s = s.mutable_difference(s, sets.make([1, 1]))
+    asserts.new_set_equals(env, sets.make([2]), s)
+
+    return unittest.end(env)
+
+mutable_difference_test = unittest.make(_mutable_difference_test)
+
 def _to_list_test(ctx):
     """Unit tests for sets.to_list."""
     env = unittest.begin(ctx)