Editing Logic programming example (section)

== Verify parent 2's statement ==
parent2_statement_is_valid(parent1_sex, child_sex, child_described_sex)

Parent 2 said "The child is a girl.  The child lied."  If Parent 2 is Male
then the only way this can be a legal statement is if the child is Female and
said they were Male.  This would mean the child is in fact a girl and the
child did in fact lie, two statements which are both true.  This corresponds
to (Male, Female, Male) being legal.

If Parent2 is Female then (Female, *, Female) are both true.  (Female, Male,
Female) is true because the first statement is false (the child is a girl) but
the second one is true (the child lied -- it said Female when it was Male).
(Female, Female, Female) is also legal since the first statement (the child is
a girl) is true but the second one is a lie (the child lied -- the child said
they were Female and they are Female).

Any other combination will be illegal.
<haskell>
parent2_statement_is_valid :: Sex -> Sex -> Sex -> Bool
parent2_statement_is_valid Male Female Male = True
parent2_statement_is_valid Female _ Female = True
parent2_statement_is_valid _ _ _ = False
</haskell>