diff --git a/pub/src/curve.cpp b/pub/src/curve.cpp
index 69061ce6a85ca9baf754f26541572bc839be92f0..319a5c5f2bebe91ff7015348af7c3c932ad372cb 100644
--- a/pub/src/curve.cpp
+++ b/pub/src/curve.cpp
@@ -178,6 +178,8 @@ void COlc::curve_val_vec( vector<double>* ret, const vector<double>& vt,
   // input: the function arguments vt[], and the context k, j
   // output: the curve evaluation result *ret[]
 {
+    if( !vt.size() )
+        throw "BUG: vectorial curve evaluation requested with empty t vector";
     cout << "DEB cvv: entry with size(vt)=" << vt.size() << "\n";
     cout << "DEB cvv: ... vt = " << vt.front() << " .. " << vt.back() << "\n";
     
diff --git a/pub/src/plot.cpp b/pub/src/plot.cpp
index 95eb80b3d2e60a852cbb07fee9fa7848c5f0ebcb..9fcb72cec4accb24e0ce9ab94074924d5595fb31 100644
--- a/pub/src/plot.cpp
+++ b/pub/src/plot.cpp
@@ -254,13 +254,17 @@ int plot_curve_refine( CPlot* plot, POlc fc, uint k, uint j, int cstyle )
 
     // refinement loop:
     for ( int iref=0; ; ++iref ) {
-        // cout << "DEB iref=" << iref << "\n";
-
+        if ( !xc.size() ){
+            if ( !iref )
+                throw "BUG: plot_curve_refine called with xc.size()=0";
+            else
+                break; // regular exit
+        }
+        cout << "DEB refinement #" << iref << ", xc.size=" << xc.size() << "\n";
         // evaluate curve for grid xc:
         fc->curve_val_vec( &yc, xc, k, j );
         // for ( uint i=0; i<xc.size(); ++i )
         // printf ("DEB xc yc %12.6g %12.6g\n", xc[i], yc[i] );
-        // cout << "DEB evaluation -> " << xc.size() << "\n";
 
         // merge xc,yc and xn,yn:
         if ( iref==0 ) {
@@ -291,13 +295,13 @@ int plot_curve_refine( CPlot* plot, POlc fc, uint k, uint j, int cstyle )
             }
             xn = xa; yn = ya;
         }
-        // cout << "DEB merger -> " << xn.size() << "\n";
+        cout << "DEB merger -> " << xn.size() << "\n";
         // for ( uint i=0; i<xn.size(); ++i )
         // printf ("DEB xn yn %12.6g %12.6g\n", xn[i], yn[i] );
 
         // end of loop
         if ( iref>=10 || xn.size()>=plot->maxpoints )
-            break;
+            break; // alternate exit: too many refinements or too many points
 
         // eliminate points outside y range (except border points):
         vector<double> xa, ya;
@@ -323,7 +327,7 @@ int plot_curve_refine( CPlot* plot, POlc fc, uint k, uint j, int cstyle )
         xa.push_back( xn.back() );
         ya.push_back( yn.back() );
         xn = xa; yn = ya;
-        // cout << "DEB within frame -> " << xn.size() << "\n";
+        cout << "DEB within frame -> " << xn.size() << "\n";
         // for ( uint i=0; i<xn.size(); ++i )
         // printf ("DEB xy %12.6g %12.6g\n", xn[i], yn[i] );
 
@@ -337,14 +341,14 @@ int plot_curve_refine( CPlot* plot, POlc fc, uint k, uint j, int cstyle )
                     (xn[i+1]-xn[i-1]);
                 if ( !plot->Y.close_enough( yn[i], yi, 0.005 ) ) {
                     xc.push_back( (xn[i-1] + xn[i])/2 );
-                    // cout << "DEB disc " << xc.back() << "\n";
+                    cout << "DEB disc " << xc.back() << "\n";
                     insert_next = true;
                     continue;
                 }
             }
             if ( insert_next ) {
                 xc.push_back( (xn[i-1] + xn[i])/2 );
-                // cout << "DEB next " << xc.back() << "\n";
+                cout << "DEB next " << xc.back() << "\n";
                 insert_next = false;
                 continue;
             }
@@ -352,7 +356,7 @@ int plot_curve_refine( CPlot* plot, POlc fc, uint k, uint j, int cstyle )
                    !plot->Y.contains(yc[i-1]) ) ||
                  ( !plot->Y.contains(yc[i]) &&
                    plot->Y.contains(yc[i-1]) ) ) {
-                // cout << "DEB edge " << xc.back() << "\n";
+                cout << "DEB edge " << xc.back() << "\n";
                 xc.push_back( (xn[i-1] + xn[i])/2 );
                 continue;
             }
@@ -362,8 +366,7 @@ int plot_curve_refine( CPlot* plot, POlc fc, uint k, uint j, int cstyle )
             for ( uint i=0; i<xc.size()-1; ++i )
                 if ( xc[i+1]<=xc[i] )
                     throw "BUG: new base points not sorted at " +
-                        S(i) + ": " + S(xc[i]) + " " +
-                        S(xc[i+1]);
+                        S(i) + ": " + S(xc[i]) + " " + S(xc[i+1]);
 
     }
 
@@ -379,7 +382,7 @@ int plot_curve_refine( CPlot* plot, POlc fc, uint k, uint j, int cstyle )
         if ( !plot->Y.contains(yn[i]) || i==xn.size()-1 ) {
             npts += xa.size();
             if ( xa.size() ){
-                // cout << "DEB segment -> " << xa.size() << "\n";
+                cout << "DEB segment -> " << xa.size() << "\n";
                 plot->addSpec(
                     true, first_seg, cstyle, xa, ya,
                     novec, fc->V[j]->z, fc->xco.str_std(), fc->yco.str_std(),