theorem Th26:
  (a,x).W = p & (a,v).W = b implies (a,(x+*(m,v))).W = (p+*(m,b))
proof
  assume (a,x).W = p & (a,v).W = b;
  then W.(a,p) = x & W.(a,b) = v by Th15,MIDSP_2:33;
  then W.(a,(p+*(m,b))) = (x+*(m,v)) by Th25;
  hence thesis by Th15;
end;
