%==========================================================================
%
%       Number of labeled rooted trees with n nodes: n^(n-1).
%       Is enumerated by EIS A000169 (1,2,9,64,625,7776,117649,2097152,...)
%
% See also:
%
% http://www.research.att.com/cgi-bin/access.cgi/as/njas/sequences/eisA.cgi?Anum=A000169
% http://www.iki.fi/~kartturi/matikka/ModFin/A000169v2.fnd
% http://www.iki.fi/~kartturi/matikka/ModFin/A000272.fnd
% http://www.iki.fi/~kartturi/matikka/ModFin/A000272R.fnd
% http://www.iki.fi/~kartturi/matikka/ModFin/A001035R.fnd
%
% http://arp.anu.edu.au/~jks/finder.html
%
% Note: The penultimate axiom specifies that if a vertex has two distinct
%       nodes below it ("<"), then they should be comparable by themselves.
%       I.e. the Hasse-diagram of these posets cannot branch downward.
%
%==========================================================================

sort {
        ELEM
        cardinality = 6
}


function {
        e:      ELEM,ELEM -> bool.
        f:      ELEM -> ELEM { cut }
}


clause {
        e(x,x) -> FALSE.                        % Irreflexive.
        e(x,y) -> FALSE = e(y,x).               % Asymmetric.
        FALSE = e(x,y); FALSE = e(y,z); e(x,z). % Transitive.
        FALSE = e(x,z); FALSE = e(y,z); x=y; e(x,y); e(y,x). % IF (x<z)&(y<z)&(x!=y) THEN x<y OR y<x.
        x=y; e(f(x),x); e(f(y),y).              % If x and y are distinct, then at least other has a predecessor.
}

setting {
        verbosity stats: full
        stack: maximal
}


end