(var1=A, var2=A), body must return 0 or NIL Antisemetric : when (var1=A, var2=B) returns -1 or T, then (var1=B, var2=A) must return +1 or NIL Transitive : when (var1=A, var2=B) returns -1 or T, and (var1=B, var2=C) returns -1 or T, then (var1=A, var2=C) must return -1 or T as well.