% P55 (**) Construct completely balanced binary trees for a given 
% number of nodes.

% cbal_tree(N,T) :- T is a completely balanced binary tree with N nodes.
% (integer, tree)  (+,?)

cbal_tree(0,nil) :- !.
cbal_tree(N,t(x,L,R)) :- N > 0,
	N0 is N - 1, 
	N1 is N0//2, N2 is N0 - N1,
	distrib(N1,N2,NL,NR),
	cbal_tree(NL,L), cbal_tree(NR,R).

distrib(N,N,N,N) :- !.
distrib(N1,N2,N1,N2).
distrib(N1,N2,N2,N1).