(*# P57 (**) Binary search trees (dictionaries)
# 
# Use the predicate add/3, developed in chapter 4 of the course, to write a
# predicate to construct a binary search tree from a list of integer numbers.
# 
# Example:
# * construct([3,2,5,7,1],T).
# T = t(3, t(2, t(1, nil, nil), nil), t(5, nil, t(7, nil, nil)))
# 
# Then use this predicate to test the solution of the problem P56.
# Example:
# * test-symmetric([5,3,18,1,4,12,21]).
# Yes
# * test-symmetric([3,2,5,7,1]).
# No*)