provide unary minus to flip a TimeVar around origin

src/lib/time/timevalue.hpp

@@ -138,6 +138,9 @@ namespace time {
       
       // Supporting multiplication with integral factor
       TimeVar& operator*= (int fact)           { t_ *= fact;  return *this; }
+      
+      // Supporting flip sign
+      TimeVar  operator-  ()         const     { return TimeVar(*this)*=-1; }
        
       // baseclass TimeValue is already totally_ordered 
     };

tests/lib/time/time-value-test.cpp

@@ -144,6 +144,11 @@ namespace test{
           gavl_time_t raw (var);
           CHECK (raw == org);
           CHECK (raw >  org - two);
+          
+          // unary minus will flip around origin
+          CHECK (zero == -var + var);
+          CHECK (zero != -var);
+          CHECK (var  == org);  // unaltered
         }