let AFV be WeakAffVect; :: thesis: for a, b, c, d being Element of AFV st a,b // c,d holds
c,d // a,b
let a, b, c, d be Element of AFV; :: thesis: ( a,b // c,d implies c,d // a,b )
assume A1: a,b // c,d ; :: thesis: c,d // a,b
c,d // c,d by Th1;
hence c,d // a,b by A1, Def1; :: thesis: verum