This site should contain more information on set theory. A discussion of the symbols for "contained," and "properly contained" should be included, as well as a discussion of power sets and the empty set. Also, it seems slightly blasphemous to state that most theories in mathematics are incomplete. The nature of mathematics is that new and interesting topics are always being analyzed and discussed, and to say that mathematics consists mostly of incomplete theories makes it sound less important than it really is. For instance, even though Christianity is widely accepted, it is still being analyzed and developed, as theologians continue to interpret the messages of the Bible and its related texts. However, saying that the study of Christianity is incomplete sounds about as wrong as saying that many theories in Mathematics are incomplete.