Estate duty : surviving spouse bequeathed usufruct - estate of first-dying insufficient to attract estate duty - whether value of the usufruct ceasing on death of surviving spouse deductible under section 4(m) of Estate Duty Act 45 of 1955 - Die Kommissaris Suid-Afrikaanse Inkomste Diens v Boedel van Wyle A I J de Beer Case number 328/99 (SCA)