Making call to add function crashs the program

I'm putting one assert that always will fail if the add function
of the Symbol is called. It's to protect of missuse.
Inserting test of constant expression class
Removing type from the union of semantic values

Also, I'm setting the expect conflicts to zero. I think it's the
default value, however I guess expliciting it is the best.
Event type test

During the creation of the test, I decided to change the type of the
symbol from CONSTANT_EXPRESSION_SYMBOL ('C') to EVENT_TYPE_SYMBOL ('V').