x1 is 10-21 of x2 in dimension/aspect x3 (default is units).
(cf. grake, mitre, snidu, stero, delno, molro, kelvo, xampo, gradu, litce, merli, centi, decti, dekto, femti, gigdo, gocti, gotro, kilto, megdo, mikri, milti, nanvi, petso, picti, terto, xatsi, xecto, xexso, zetro)