[Stolen from a Perl6 programming challenge]
On the mythical island of Smul, people suffer from a rare genetic
disorder that make them either tell the truth all the time, or lie
all the time. These are the only two types of people on the island,
known as knights and knaves, respectively.
Write a program that takes as input a number of utterances by
islanders, and outputs for each person whether that person is a
knight or a knave. If there is no possible assignment that works,
the program should report that no solution exists. In the case of
multiple solutions, the program should report every possible
solution.
The islanders can make four different classes of utterances:
X is a knight.
X is a knave.
X and Y are of the same type.
X and Y are of different types.
(Here, X and Y are used as metavariables, of course, and can in
fact be any name of an islander.)
Islanders can refer to each other. The same islander can make
several utterances. If an islander mentions another islander
that doesn't say anything, your program should consider the entire
input to be erroneous.
Here are a few examples:
A: A is a knight.
Both a knight and a knave would assert the same thing. So this input
has two solutions.
B: B is a knave.
Neither a knight or a knave would ever say this about themselves. So
this input allows no solution.
C: C and D are of the same type.
D: D and C are of different types.
Here, the two islanders are contradicting each other, so one of them
must be a knight and the other a knave. But this is exactly what D is
saying, so D is the knight. One solution.
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