The class tt(ModBase) defines the interface for modalities that is available
to other parts of s(), in particular to members of the class tt(Loop)
(cf. section ref(LOOP)). The interface provides the following members:
    itemization(
    itt(double sensitivity(size_t idx)): the modality's sensitivity given a
        bi-rad category tt(idx). For modalities that are independent of bi-rad
        categories the tt(idx) argument is ignored;

    itt(double specificity(double age)): the modality's specificity given an
        age. For modalities that are independent of age the tt(age) argument
        is ignored;

    itt(uint16_t cost()): returns the costs associated with using the
        modality;
    
    itt(uint16_t nr() const): returns an identifying order-number of the
        modality; The order number is used, e.g., when updating the number of
        false negative decisions during screening loops;

    itt(void resetCounters(size_t nRounds)): clears the vector counting the
        number of times modalities are used by the screening rounds and resets
        the number of encountered false positives to 0;

    itt(void count(size_t round)): increments the usage count for the
        specified screening round;

    itt(void falsePositive()): increments the number of false positive
        decisions; 

    itt(std::string const &id()): returns the name (e.g., MRI, Mammo) of the
        modality;

    itt(double const *dose()): returns the array of radiation doses associated
        with the modality or 0 if there are no associated radiation doses;

    itt(double dose(uint16_t idx)): returns the radiation dose of a specified
        bi-rad category or 0 if there are no associated radiation doses.
    )
