theorem Th8:
  a<>b & p,q // a,b & a,b // r,s implies p,q // r,s
proof
  assume that
A1: a<>b and
A2: p,q // a,b and
A3: a,b // r,s;
  a,b // p,q by A2,Th6;
  hence thesis by A1,A3,Def1;
end;
