sorts two Div structures from the identifiers they share
x1r,x2r = reorder(x1,x2);
two Div structures
two Div structures containing the observations common to x1 and x2 according to x1.i et x2.i, and in the same order for x1r and x2r (x1r.i=x2r.i); the observations not shared by x1 and x2 are dropped