diff --git a/persistent-data/run_set.h b/persistent-data/run_set.h
index 707dde0..4ecd137 100644
--- a/persistent-data/run_set.h
+++ b/persistent-data/run_set.h
@@ -87,6 +87,33 @@ namespace base {
 			return runs_.end();
 		}
 
+		void negate() {
+			rset replacement;
+
+			if (runs_.begin() == runs_.end())
+				replacement.insert(run<T>());
+			else {
+				auto b = runs_.begin();
+				maybe last = b->end_;
+
+				if (b->begin_)
+					replacement.insert(run<T>(maybe(), *(b->begin_)));
+
+				++b;
+				while (b != runs_.end()) {
+					replacement.insert(run<T>(last, b->begin_));
+					last = b->end_;
+					++b;
+				}
+
+				if (last)
+					replacement.insert(run<T>(last, maybe()));
+
+			}
+
+			runs_ = replacement;
+		}
+
 	private:
 		typedef typename run<T>::maybe maybe;
 
@@ -104,7 +131,7 @@ namespace base {
 			return maybe(std::max<T>(*m1, *m2));
 		}
 
-		std::set<run<T>, compare_begin> runs_;
+		rset runs_;
 	};
 }
 
diff --git a/unit-tests/run_set_t.cc b/unit-tests/run_set_t.cc
index e6f26bd..02ce070 100644
--- a/unit-tests/run_set_t.cc
+++ b/unit-tests/run_set_t.cc
@@ -16,6 +16,18 @@ namespace {
 			(arg.end_ == static_cast<unsigned>(e));
 	}
 
+	MATCHER(EqAll, "") {
+		return !arg.begin_ && !arg.end_;
+	}
+
+	MATCHER_P(EqOpenBegin, e, "") {
+		return !arg.begin_ && (arg.end_ == static_cast<unsigned>(e));
+	}
+
+	MATCHER_P(EqOpenEnd, b, "") {
+		return (arg.begin_ == static_cast<unsigned>(b)) && !arg.end_;
+	}
+
 	class RunSetTests : public Test {
 	};
 }
@@ -176,4 +188,46 @@ TEST_F(RunSetTests, merge_discrete_sets)
 	ASSERT_THAT(++(++(rs1.begin())), Eq(rs1.end()));
 }
 
+TEST_F(RunSetTests, negate_empty)
+{
+	run_set<unsigned> rs;
+	rs.negate();
+	ASSERT_THAT(*rs.begin(), EqAll());
+	ASSERT_THAT(++rs.begin(), Eq(rs.end()));
+}
+
+TEST_F(RunSetTests, negate_single)
+{
+	run_set<unsigned> rs;
+	rs.add(5, 10);
+	rs.negate();
+	ASSERT_THAT(*rs.begin(), EqOpenBegin(5));
+	ASSERT_THAT(*(++rs.begin()), EqOpenEnd(10));
+	ASSERT_THAT(++(++rs.begin()), Eq(rs.end()));
+}
+
+TEST_F(RunSetTests, negate_double)
+{
+	run_set<unsigned> rs;
+	rs.add(5, 10);
+	rs.add(15, 20);
+	rs.negate();
+	ASSERT_THAT(*rs.begin(), EqOpenBegin(5));
+	ASSERT_THAT(*(++rs.begin()), EqRun(10, 15));
+	ASSERT_THAT(*(++(++rs.begin())), EqOpenEnd(20));
+	ASSERT_THAT(++(++(++rs.begin())), Eq(rs.end()));
+}
+
+TEST_F(RunSetTests, negate_negate)
+{
+	run_set<unsigned> rs;
+	rs.add(5, 10);
+	rs.add(15, 20);
+	rs.negate();
+	rs.negate();
+	ASSERT_THAT(*rs.begin(), EqRun(5, 10));
+	ASSERT_THAT(*(++rs.begin()), EqRun(15, 20));
+	ASSERT_THAT(++(++rs.begin()), Eq(rs.end()));
+}
+
 //----------------------------------------------------------------