This is a small example of the 'forall'/'require' clause used to verify that
all of the elements a list meet some requirement.  This example is done in
both forward-chaining and backward-chaining rules.

The forward-chaining and backward-chaining rules are in two different .krb
files showing examples of use of the 'forall'/'require' clause in both cases.

These rules find all people who have no step brothers or sisters.

    >>> import test
    >>> test.fc_test()
    ... prints out everybody that doesn't have any step brothers or sisters ...
    >>> test.bc_test()
    ... prints out everybody that doesn't have any step brothers or sisters ...

