The tt(Densities) object provides access to information about the breast
densities (cf. url(bi-rads info)
  (https://radiologyassistant.nl/breast/bi-rads/bi-rads-for-mammography-and-ultrasound-2013)).
The American College of Radiology (ACR) distinguishes four breast densitiy
categories:
    itemization(
    it() a: the breasts are almost entirely fatty
    it() b: there are scattered areas of fibroglandular density
    it() c: the breasts are heterogeneously dense, which may obscure small
        masses
    it() d: the breasts are extremely dense, which lowers the sensitivity of
mammograph
    )
(see also url(the ACR reference card)
(https://www.acr.org/-/media/ACR/Files/RADS/BI-RADS/BIRADS-Reference-Card.pdf))

In the s() configuration file the distributions of probabilities of these four
categories by age groups are provided by the tt(breastDensities:)
specifications, e.g.,
    verb(
                                    bi-rad classification
                                 ----------------------------
                     agegroup     a       b       c       d
 breastDensities:    0  - 40     0.05    0.30    0.48    0.17
 ...
 breastDensities:    70 - *      0.18    0.54    0.26    0.02
    )

Age groups consist of an initial age and an upper limit. The upper limit is
not considered part of the age range, but becomes the initial age of the next
range. The final age range may use age specification tt(*), indicating the
maximum possible age of simulated cases. Specifications following a
specification using ending age tt(*) are ignored.  S() checks whether the
bi-rad probabilities per age group add to 1.00, and whether the ending age of
an age group is equal to the initial age of the next age group.

At the initialization phase of tt(Loop::genCases) (cf. section ref(GENCASES)
breast densitiy indices (0..3) corresponding to bi-rad classifications a..d
are determined for the simulated case's screening ages. The member
tt(Densities::indices) generates a random value from the uniform random
distribution and uses that probability to determine the bi-rad classification
for the various screening ages, returning a vector containing the bi-rad
classifications (0..3) for each screening round.
