Attempt to document List.categorize, most of the text from https://raw.githubusercontent.com/perl6/specs/master/S32-setting-library/Containers.pod

gistfile1.txt

=head2 routine categorize

Defined as:

    multi sub    categorize(&mapper, *@values) returns Hash:D
    multi method categorize(List:D: &mapper)   returns Hash:D

Usage:

    categorize MAPPER, LIST
    LIST.categorize(MAPPER)

Transforms a list into a hash of arrays, based on categories determined
by a mapper function. The mapper receives a value as its argument and
returns a list of categories. In the resulting hash, each key will be a
category and its value will be an array of corresponding values from the
original list.

Note that, unlike L<classify>, which assumes that the return value
of the mapper is a single value, C<categorize> always assumes that
the return value of the mapper is a list of categories that are
appropriate to the current value.

Example:

    sub mapper(Int $i) returns List {
        $i %% 2 ?? 'even' !! 'odd',
        $i.is-prime ?? 'prime' !! 'not prime'
    }
    say categorize &mapper, (1, 7, 6, 3, 2);   # {even => [2], odd => [7 3], prime => [7 3 2]}