Adds a new variable with no value specified. If the variable already exists in the table, that variable instance is returned. Otherwise a new Variable instance is created with the variable factory.
Adds a new variable with specified value. If the variable already exists, its value is updated with the new value. If the variable exists and is a constant, the value is not updated and no error is reported. If the variable does not exist, it is created using the variable factory of the jep instance specified with the Init(JepInstance) method.