diff --git a/fourindex.h b/fourindex.h
index 8fda646..15530ee 100644
--- a/fourindex.h
+++ b/fourindex.h
@@ -719,7 +719,7 @@ public:
 
 //we cannot return reference due to the possible sign change
 //stored values are for i>j a>b
-        inline T operator() (unsigned int i, unsigned int j, unsigned int a, unsigned int b)
+        inline T operator() (unsigned int i, unsigned int j, unsigned int a, unsigned int b) const
 {
 #ifdef DEBUG
 if(i<1||i>nocc ||j<1||j>nocc|| a<1||a>nvrt||b<1||b>nvrt) laerror("T2ijab_aces fourindex out of range")
diff --git a/matexp.h b/matexp.h
index 66a7f97..13be351 100644
--- a/matexp.h
+++ b/matexp.h
@@ -261,4 +261,7 @@ for(int i=1; i<=(1<<power); ++i) //unfortunatelly, here we have to repeat it man
 return result;
 }
 
+//@@@ power series matrix logarithm?
+//@@@ interface to expokit?
+
 #endif