This module provides a number of list utility functions, all of which take
an initial code block to control their behaviour. They are variations on
similar core perl or List::Util functions of similar names, but which use
the block to control their behaviour. For example, the core Perl function
sort takes a list of values and returns them, sorted into order by their
string value. The sort_by function sorts them according to the string value
returned by the extra function, when given each value.
