modified from BASIC code in:

Byers, J.A. 1993. Randomization algorithms in BASIC for experimental
     design. Computers in Biology and Medicine 23:167-176.

other related programs:
Byers, J.A. 1991. BASIC algorithms for random sampling and treatment
     randomization. Computers in Biology and Medicine 21:69-77.

Byers, J.A. 1996. Random selection algorithms for spatial and temporal
     sampling. Computers in Biology and Medicine 26:41-52.