The praelegatum is a concursus causarum-problem of universal succession and particular succession die cedente.
Since if the will failed, the legacy failed with it, it ist the legacy (and not the testamentary succession) which is
effected by the cc-problem. The legacy is void.
If the legacy failed in the cc, there can be no vesting of legacy (no expectancy): Pap. D 31, 75, 1. However, when
the heir/legatee for any reason could not or did not accept the inheritance, the legacy is good titel (Scaev. D 32, 32).
Henceforward: when omission of the inheritance gives effect to the legacy in the heredi a semet ipso-part (because
there is no cc-situation), the void part of the legacy is not a case of the regula Catoniana. Also: When the
heir/legatee is charged with a trust to make over the inheritance to some one else, the legacy in the heredi a semet
ipso-part is good titel, since this case different from the cc-problem (Jul. D 35, 2, 86).