diff --git a/Makefile b/Makefile
index 4fcd5dc..d27ef2f 100644
--- a/Makefile
+++ b/Makefile
@@ -24,8 +24,9 @@ OBJS=\
     # -fno-rtti: libf does not use runtime type information.
     # -nodefaultlibs: libf must not depend on the C++ stdlib.  It does depend
     #                 on libc, so we use must use -lc linker option.
-CXX = clang++-4.0 -std=c++1z -fno-exceptions -fno-rtti -nodefaultlibs -fPIC \
-    -O2
+#CXX = clang++-7 -std=c++1z -fno-exceptions -fno-rtti -nodefaultlibs -fPIC \
+       -O2
+CXX = g++ -std=c++2a -fno-exceptions -fno-rtti -nodefaultlibs -fPIC -O2
 COPTS = -fPIC 
 CLIBS = -lc -lgc
 CLIB = $(OBJS)
diff --git a/examples/Makefile b/examples/Makefile
index 745b28b..ae2f3d6 100644
--- a/examples/Makefile
+++ b/examples/Makefile
@@ -1,5 +1,5 @@
 
-CC = clang++-4.0 -std=c++1z -fno-exceptions -fno-rtti -nodefaultlibs 
+CC = g++ -std=c++2a -fno-exceptions -fno-rtti -nodefaultlibs 
 COPTS = -fPIC -O2
 CLIBS = -lc -lgc
 CLIB = $(OBJS)
diff --git a/fseq.cpp b/fseq.cpp
index 8136c59..8d13f7c 100644
--- a/fseq.cpp
+++ b/fseq.cpp
@@ -827,7 +827,7 @@ extern PURE Frag _seq_peek_front(Seq s)
         case SINGLE:
         {
             const Single &ss = s;
-            return ss.t[0];
+            return static_cast<const Frag&>(ss.t[0]);
         }
         case DEEP:
         {
@@ -837,22 +837,22 @@ extern PURE Frag _seq_peek_front(Seq s)
                 case DIG_1:
                 {
                     const Dig1 &r1 = sd.l;
-                    return r1.t[0];
+                    return static_cast<const Frag&>(r1.t[0]);
                 }
                 case DIG_2:
                 {
                     const Dig2 &r2 = sd.l;
-                    return r2.t[0];
+                    return static_cast<const Frag&>(r2.t[0]);
                 }
                 case DIG_3:
                 {
                     const Dig3 &r3 = sd.l;
-                    return r3.t[0];
+                    return static_cast<const Frag&>(r3.t[0]);
                 }
                 case DIG_4:
                 {
-                    const Dig4 r4 = sd.l;
-                    return r4.t[0];
+                    const Dig4 &r4 = sd.l;
+                    return static_cast<const Frag&>(r4.t[0]);
                 }
                 default:
                     error_bad_tree();
@@ -909,7 +909,7 @@ static Seq seq_push_back(Seq s, Tree t)
                     const Dig4 &s4 = sd.r;
                     if (index(sd.m) == NIL && index(sd.l) == DIG_1)
                     {
-                        Dig1 l1 = sd.l;
+                        Dig1 l1(sd.l);
                         return deep(dig3(l1.t[0], s4.t[0], s4.t[1]),
                             empty(), dig3(s4.t[2], s4.t[3], t));
                     }
@@ -1043,7 +1043,7 @@ extern PURE Frag _seq_peek_back(Seq s)
         case SINGLE:
         {
             const Single &ss = s;
-            return ss.t[0];
+            return static_cast<const Frag&>(ss.t[0]);
         }
         case DEEP:
         {
@@ -1053,22 +1053,22 @@ extern PURE Frag _seq_peek_back(Seq s)
                 case DIG_1:
                 {
                     const Dig1 &r1 = sd.r;
-                    return r1.t[0];
+                    return static_cast<const Frag&>(r1.t[0]);
                 }
                 case DIG_2:
                 {
                     const Dig2 &r2 = sd.r;
-                    return r2.t[1];
+                    return static_cast<const Frag&>(r2.t[1]);
                 }
                 case DIG_3:
                 {
                     const Dig3 &r3 = sd.r;
-                    return r3.t[2];
+                    return static_cast<const Frag&>(r3.t[2]);
                 }
                 case DIG_4:
                 {
                     const Dig4 &r4 = sd.r;
-                    return r4.t[3];
+                    return static_cast<const Frag&>(r4.t[3]);
                 }
                 default:
                     error_bad_tree();
@@ -1112,14 +1112,14 @@ static Seq seq_append(Seq s, Tree *m, size_t m_len, Seq t)
                         s = seq_push_back(s, m[i]);
                     if (index(t) == SINGLE)
                     {
-                        Single ts = (t);
+                        Single ts(t);
                         s = seq_push_back(s, ts.t[0]);
                     }
                     return s;
                 case DEEP:
                 {
                     const Deep &sd = s;
-                    Deep td = (t);
+                    Deep td(t);
                     m_len = seq_append_middle(sd.r, m, m_len, td.l);
                     Seq u = seq_append(sd.m, m, m_len, td.m);
                     return deep(sd.l, u, td.r);