Index: mttroot/mtt/lib/cr/r/slin.cr
==================================================================
--- mttroot/mtt/lib/cr/r/slin.cr
+++ mttroot/mtt/lib/cr/r/slin.cr
@@ -1,6 +1,8 @@
 %DESCRIPTION Sensitivity version of lin
+
+in "$MTT_CRS/r/lin.cr";
 
 % One port version for historical reasons
 OPERATOR slin;
 FOR ALL gain_causality, gain, causality, input, other_causality
 LET slin(gain_causality, gain, other_causality, 1, input, causality, 1)
@@ -18,15 +20,25 @@
                input, causality, 1); 
 
 
 % Port 2 - sensitivity
 FOR ALL gain_causality, gain, sgain, causality, input, sinput, other_causality
+SUCH THAT sgain NEQ 0
 LET slin(gain_causality, gain, sgain, other_causality, 2, 
        input, causality, 1,
        sinput, causality, 2)
          = lin(gain_causality, gain, other_causality, 1, 
                sinput, causality, 1)
          + sgain*df(lin(gain_causality, gain, other_causality, 1, 
                input, causality, 1), gain); 
 
+%% Don't compute the derivative in this case
+FOR ALL gain_causality, gain, sgain, causality, input, sinput, other_causality
+SUCH THAT sgain EQ 0
+LET slin(gain_causality, gain, sgain, other_causality, 2, 
+       input, causality, 1,
+       sinput, causality, 2)
+         = lin(gain_causality, gain, other_causality, 1, 
+               sinput, causality, 1);
+         
 END;;