% problem-set/puzzles/carroll/letters.ver1.clauses 
% created : 07/19/88
% revised : 07/19/88 

% description:
%
% This run solves "The Letters Puzzle" using
% UR-resolution, hyper-resolution, and binary resolution.     
%
%                		The Letters Puzzle
%
% (1) All the dated letters in this room are written on blue paper.
% (2) None of them are in black ink, except those that are written
%     in the third person.
% (3) I have not filed any of them that I can read.
% (4) None of them, that are written on one sheet, are undated.
% (5) All of them, that are not crossed, are in black ink.
% (6) All of them, written by Brown, begin with "Dear Sir."
% (7) All of them, written on blue paper, are filed.
% (8) None of them,  written on more than one sheet, are crossed.
% (9) None of them, that begin with "Dear Sir," are written in
%     third person.
%     Find out everything that you can about the letters.
%						-Lewis Carroll, Symbolic Logic, p. 175 

% representation:
%
                  

-DATED | ON_BLUE_PAPER.
-IN_THIRD_PERSON | IN_BLACK_INK.
IN_THIRD_PERSON | -IN_BLACK_INK.
-CAN_BE_READ | -FILED.
-ON_ONE_SHEET | DATED.
CROSSED | IN_BLACK_INK.
-BY_BROWN | BEGINS_WITH_DEAR_SIR.
-ON_BLUE_PAPER | FILED.
ON_ONE_SHEET | -CROSSED.
-BEGINS_WITH_DEAR_SIR | -IN_THIRD_PERSON.