theorem
  <*a,b*> = <*c,d*> implies a = c & b = d
proof
  assume
A1: <*a,b*> = <*c,d*>;
  thus a = <*a,b*>.1
    .= c by A1,Th44;
  thus b = <*a,b*>.2
    .= d by A1,Th44;
end;
