theorem Th26:
  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,Th23;
  hence thesis by A1,A3,Def11;
end;
