theorem Th25:
  [*x1,x2,x3,x4*] *' = [*x1,-x2,-x3,-x4*]
proof
  set c = [*x1,x2,x3,x4*];
A1: Rea c = x1 by QUATERNI:23;
A2: Im1 c = x2 by QUATERNI:23;
A3: Im2 c = x3 by QUATERNI:23;
  Im3 c = x4 by QUATERNI:23;
  hence thesis by A1,A2,A3,QUATERNI:43;
end;
