theorem
  for x,y being FinSequence of COMPLEX st len x=len y holds
  |(x+y,x+y)| = |(x,x)| + 2*Re(|(x,y)|) + |(y,y)|
proof
  let x,y be FinSequence of COMPLEX;
  set z=|(x, y)|;
  assume len x=len y;
  then |(x+y, x+y)| =|(x, x)| + |(x, y)| + |(y, x)| + |(y, y)| by Th62
    .=|(x, x)| + |(x, y)| + (|(x, y)|)*' + |(y, y)| by Th64
    .=|(x, x)| + (z+z*') + |(y, y)|
    .=|(x, x)| + 2*Re(|(x, y)|) + |(y, y)| by Th20;
  hence thesis;
end;
