-  user_property(?UName, ?Property) is nondet
 - True when Property is a property of user. In a real application this
should of course be a proper persistent database and passwords
should be properly hashed.
 
Undocumented predicates
The following predicates are exported, but not or incorrectly documented.
-  authenticate_user(Arg1, Arg2, Arg3)
 
-  add_user(Arg1, Arg2, Arg3)
 
-  current_user(Arg1)
 
-  current_user(Arg1, Arg2)
 
-  use_default_db
 
-  set_user_property(Arg1, Arg2)
 
-  assert_user_property(Arg1, Arg2)
 
-  retract_user_property(Arg1, Arg2)
 
-  retractall_user_property(Arg1, Arg2)