diff --git a/app/StaMain.cc b/app/StaMain.cc
index 6cb54604..2366fc5e 100644
--- a/app/StaMain.cc
+++ b/app/StaMain.cc
@@ -117,8 +117,7 @@ staTclAppInit(Tcl_Interp *interp)
 
   // Import exported commands from sta namespace to global namespace.
   Tcl_Eval(interp, "sta::define_sta_cmds");
-  const char *export_cmds = "namespace import sta::*";
-  Tcl_Eval(interp, export_cmds);
+  Tcl_Eval(interp, "namespace import sta::*");
 
   if (!findCmdLineFlag(argc, argv, "-no_init"))
     sourceTclFileEchoVerbose(init_filename, interp);
diff --git a/graph/Graph.cc b/graph/Graph.cc
index 333a3d18..6f0eb1ff 100644
--- a/graph/Graph.cc
+++ b/graph/Graph.cc
@@ -146,7 +146,8 @@ Graph::vertexAndEdgeCounts(const Instance *inst,
 	while (set_iter.hasNext()) {
 	  TimingArcSet *arc_set = set_iter.next();
 	  LibertyPort *to_port = arc_set->to();
-	  if (network_->findPin(inst, to_port)) {
+	  if (network_->findPin(inst, to_port)
+	      && filterEdge(arc_set)) {
 	    if (dir->isBidirect()) {
 	      // Internal path from bidirect output back into the instance.
 	      edge_count += 2;
@@ -285,9 +286,10 @@ Graph::makePortInstanceEdges(const Instance *inst,
     TimingArcSet *arc_set = timing_iter.next();
     LibertyPort *from_port = arc_set->from();
     LibertyPort *to_port = arc_set->to();
-    if (from_to_port == nullptr
-	|| from_port == from_to_port
-	|| to_port == from_to_port) {
+    if ((from_to_port == nullptr
+	 || from_port == from_to_port
+	 || to_port == from_to_port)
+	&& filterEdge(arc_set)) {
       Pin *from_pin = network_->findPin(inst, from_port);
       Pin *to_pin = network_->findPin(inst, to_port);
       if (from_pin && to_pin) {
diff --git a/graph/Graph.hh b/graph/Graph.hh
index 393fcc6b..05f79c87 100644
--- a/graph/Graph.hh
+++ b/graph/Graph.hh
@@ -220,6 +220,8 @@ protected:
   void deleteFloats(float *floats,
 		    ObjectIndex count);
   void removeDelayAnnotated(Edge *edge);
+  // User defined predicate to filter graph edges for liberty timing arcs.
+  virtual bool filterEdge(TimingArcSet *) const { return true; }
 
   VertexPool *vertices_;
   EdgePool *edges_;
diff --git a/tcl/Util.tcl b/tcl/Util.tcl
index 699a5e36..d1bbf8ab 100644
--- a/tcl/Util.tcl
+++ b/tcl/Util.tcl
@@ -391,10 +391,19 @@ proc check_percent { cmd_arg arg } {
 ################################################################
 
 # The builtin Tcl "source" and "unknown" commands are redefined by sta.
-# This rename provices a mechanism to refer to the original TCL
+# This rename provides a mechanism to refer to the original TCL
 # command.
 rename source builtin_source
-rename unknown builtin_unknown
+
+# This seems to be a common build problem.
+# It happens with incomplete installs of libtcl that can't find the
+# corresponding init.tcl file.
+if { [info procs unknown] == {} } {
+  puts "TCL installation problem. proc unknown not defined. Check tcl_libraries $tcl_libraries for init.tcl"
+  proc unknown { args } { return -code error "invalid command name" }
+} else {
+  rename unknown builtin_unknown
+}
 
 # Numeric expressions eval to themselves so braces aren't required
 # around bus names like foo[2] or foo[*].