|- !A. {x | x IN A} = A
|- P / [] = P
|- !s t P. APPEND s t IN TRACES P ==> s IN TRACES P /\ t IN TRACES (P / s)
|- !s t P. APPEND s t IN TRACES P ==> (P / APPEND s t = (P / s) / t)
|- !c P. c IN ALPHA P ==> ((c --> P) / [c] = P)
|- !c P B. WELL_DEF_ALPHA B P /\ c IN B ==> (choice B P / [c] = P c)