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
  b,c // b9,c9 & a,d // b,c & a,d9 // b9,c9 implies d = d9
proof
  assume that
A1: b,c // b9,c9 and
A2: a,d // b,c and
A3: a,d9 // b9,c9;
  b9,c9 // b,c by A1,Th3;
  then a,d // b9,c9 by A2,Def1;
  then a,d // a,d9 by A3,Def1;
  hence thesis by Th4;
end;
