for en mer omfattende diskusjon, se joint committee on taxation, current law and background on federal tax provisions relating to retirement savings incentives, health and long-term care, and estate and gift taxes (jcx-29-99), 15. juni 1999
boken "joint committee on taxation, present law and background on federal tax provisions relating to retirement savings incentives, health and long-term care, and estate and gift taxes" har mer informasjon om dette
boken "om mus og mennesker" har mer informasjon om dette