theorem Th67:
  p in q & r <> p & r <> q & s <> p & s <> q implies (T,p,q)incl.(r,s) = [r,s]
  proof assume
A1: p in q & r <> p & r <> q & s <> p & s <> q;
    set X = dom T;
    set i = id X;
    set f = Swap(i,p,q);
    set h = [:f,f:];
    set Y = (succ q)\p;
    thus (T,p,q)incl.(r,s) = [f.r,f.s] by A1,Th63
    .= [i.r,f.s] by A1,Th33 .= [i.r,i.s] by A1,Th33
    .= [r,i.s]
    .= [r,s];
  end;
