reserve AFV for WeakAffVect;
reserve a,b,c,d,e,f,a9,b9,c9,d9,f9,p,q,r,o,x99 for Element of AFV;

theorem Th7:
  a,b // c,d implies b,a // d,c
proof
  assume
A1: a,b // c,d;
  a,a // c,c by Th6;
  hence thesis by A1,Def1;
end;
