theorem
  c1.|.(c2 - c3) = c1.|.c2 - c1.|.c3
proof
  c1.|.(c2 - c3)=c1 * (c2*' - c3*') by QUATERNI:56
    .=c1 * c2*' - c1 * c3*' by Th24;
  hence thesis;
end;
