x1 is 1x10**18 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, zepti, zetro.