Added comment why derivative of if expression must be allowed even th…

…ough it is non-existent.

git-svn-id: https://openmodelica.org/svn/OpenModelica/trunk@2501 f25d12d1-65f4-0310-ae8a-bbce733d8d8e

Compiler/Derive.mo

@@ -540,10 +540,12 @@ algorithm
       then
         Exp.RELATION(e1_1,rel,e2_1);
 
+	// if expression does not have derivative, but present here so linear equations can be 
+	// identified by calculation of jacobian.
     case (Exp.IFEXP(expCond = e1,expThen = e2,expElse = e3),tv)
       equation 
         e2_1 = differentiateExp(e2, tv);
-        e3_1 = differentiateExp(e3, tv);
+        e3_1 = differentiateExp(e3, tv); 
       then
         Exp.IFEXP(e1,e2_1,e3_1);