(*# P55 (**) Construct completely balanced binary trees
# 
# In a completely balanced binary tree, the following property holds for every
# node: The number of nodes in its left subtree and the number of nodes in its
# right subtree are almost equal, which means their difference is not greater
# than one.
# 
# Write a function cbal-tree to construct completely balanced binary trees for a
# given number of nodes. The predicate should generate all solutions via
# backtracking. Put the letter 'x' as information into all nodes of the tree.
# 
# Example:
# * cbal-tree(4,T).
# T = t(x, t(x, nil, nil), t(x, nil, t(x, nil, nil))) ;
# T = t(x, t(x, nil, nil), t(x, t(x, nil, nil), nil)) ;
# etc......No*)