diff --git a/src/ripple/app/paths/impl/BookStep.cpp b/src/ripple/app/paths/impl/BookStep.cpp
index 7f046233c7..498cc3e574 100644
--- a/src/ripple/app/paths/impl/BookStep.cpp
+++ b/src/ripple/app/paths/impl/BookStep.cpp
@@ -17,6 +17,7 @@
 */
 //==============================================================================
 
+#include <ripple/app/paths/impl/FlatSets.h>
 #include <ripple/app/paths/impl/Steps.h>
 #include <ripple/app/paths/Credit.h>
 #include <ripple/app/paths/NodeDirectory.h>
@@ -729,8 +730,7 @@ BookStep<TIn, TOut, TDerived>::revImp (
         auto const r = forEachOffer (sb, afView, prevStepRedeems, eachOffer);
         boost::container::flat_set<uint256> toRm = std::move(std::get<0>(r));
         std::uint32_t const offersConsumed = std::get<1>(r);
-        ofrsToRm.insert (boost::container::ordered_unique_range_t{},
-            toRm.begin (), toRm.end ());
+        SetUnion(ofrsToRm, toRm);
 
         if (offersConsumed >= maxOffersToConsume_)
         {
@@ -891,8 +891,7 @@ BookStep<TIn, TOut, TDerived>::fwdImp (
         auto const r = forEachOffer (sb, afView, prevStepRedeems, eachOffer);
         boost::container::flat_set<uint256> toRm = std::move(std::get<0>(r));
         std::uint32_t const offersConsumed = std::get<1>(r);
-        ofrsToRm.insert (boost::container::ordered_unique_range_t{},
-            toRm.begin (), toRm.end ());
+        SetUnion(ofrsToRm, toRm);
 
         if (offersConsumed >= maxOffersToConsume_)
         {
diff --git a/src/ripple/app/paths/impl/FlatSets.h b/src/ripple/app/paths/impl/FlatSets.h
new file mode 100644
index 0000000000..0cf5ce8b86
--- /dev/null
+++ b/src/ripple/app/paths/impl/FlatSets.h
@@ -0,0 +1,48 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of rippled: https://github.com/ripple/rippled
+    Copyright (c) 2019 Ripple Labs Inc.
+
+    Permission to use, copy, modify, and/or distribute this software for any
+    purpose  with  or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#ifndef RIPPLE_APP_PATHS_IMPL_FLAT_SETS_H_INCLUDED
+#define RIPPLE_APP_PATHS_IMPL_FLAT_SETS_H_INCLUDED
+
+#include <boost/container/flat_set.hpp>
+
+namespace ripple {
+
+/** Given two flat sets dst and src, compute dst = dst union src
+
+    @param dst set to store the resulting union, and also a source of elements for the union
+    @param src second source of elements for the union
+ */
+template <class T>
+void
+SetUnion(
+    boost::container::flat_set<T>& dst,
+    boost::container::flat_set<T> const& src)
+{
+    if (src.empty())
+        return;
+
+    dst.reserve(dst.size() + src.size());
+    dst.insert(
+        boost::container::ordered_unique_range_t{}, src.begin(), src.end());
+}
+
+}  // namespace ripple
+
+#endif
diff --git a/src/ripple/app/paths/impl/StrandFlow.h b/src/ripple/app/paths/impl/StrandFlow.h
index 205dcfe416..c6551012a2 100644
--- a/src/ripple/app/paths/impl/StrandFlow.h
+++ b/src/ripple/app/paths/impl/StrandFlow.h
@@ -23,6 +23,7 @@
 #include <ripple/app/paths/Credit.h>
 #include <ripple/app/paths/Flow.h>
 #include <ripple/app/paths/impl/AmountSpec.h>
+#include <ripple/app/paths/impl/FlatSets.h>
 #include <ripple/app/paths/impl/FlowDebugInfo.h>
 #include <ripple/app/paths/impl/Steps.h>
 #include <ripple/basics/Log.h>
@@ -506,8 +507,7 @@ flow (PaymentSandbox const& baseView,
                 sb, *strand, remainingIn, remainingOut, j);
 
             // rm bad offers even if the strand fails
-            ofrsToRm.insert (boost::container::ordered_unique_range_t{},
-                f.ofrsToRm.begin (), f.ofrsToRm.end ());
+            SetUnion(ofrsToRm, f.ofrsToRm);
 
             if (f.ter != tesSUCCESS || f.out == beast::zero)
                 continue;
@@ -589,8 +589,7 @@ flow (PaymentSandbox const& baseView,
                         // view
         if (!ofrsToRm.empty ())
         {
-            ofrsToRmOnFail.insert (boost::container::ordered_unique_range_t{},
-                ofrsToRm.begin (), ofrsToRm.end ());
+            SetUnion(ofrsToRmOnFail, ofrsToRm);
             for (auto const& o : ofrsToRm)
             {
                 if (auto ok = sb.peek (keylet::offer (o)))