From the problem statement, it seems to me that the setup indicates
only that the citizens are consistent in their truth-telling (or
lying): each citizen is either a truth-teller or a liar.
Percival
Llewellyn
Consequence
T
T
contradiction: "at least one of us is a liar"
T
L
possible! In which case, Llewellyn is the liar.
L
T
contradiction: not("at least one of us is a liar")
L
L
contradiction: not("at least one of us is a liar")
Hence, the only possibility is that Percival is a truth-teller, and Llewellyn is the liar.
Website maintained by Andy Long.
Comments appreciated.