"accrual duration given power and follow-up time", {
alpha = 0.025, typeAlphaSpending = "sfOF",
followupTime = 18, fixedFollowup = FALSE)
"follow-up time given power and accrual duration", {
followupTime = NA, fixedFollowup = FALSE)
"absolute accrual intensity given power, accrual duration, follow-up time,

"accrual duration given power and follow-up time"
"follow-up time given power and accrual duration"
"absolute accrual intensity given power, accrual duration, follow-up time,
  and relative accrual intensity"

absolute accrual intensity given power, accrual duration, follow-up time,
  and relative accrual intensity