Can the value of S be a negative number?

[LittleBeannie]

The following 2 calls of gsSurv give different outputs. Their differences lies in the values of S. I am wondering if we can input negative values to S.

According to the help file, we have S: a scalar or vector of durations of piecewise constant event rates specified in rows of lambda, eta and etaE; this is NULL if there is a single event rate per stratum (exponential failure) or length of the number of rows in lambda minus 1, otherwise.

library(gsDesign)

gsSurv(
  k = 4, sfl = sfPower, sflpar = .5, lambdaC = log(2) / c(6, 9), hr = .5,
  eta = log(2) / c(40, 50), S = 5, gamma = 1, T = 36, minfup = 12
) |> gsBoundSummary()

gsSurv(
  k = 4, sfl = sfPower, sflpar = .5, lambdaC = log(2) / c(6, 9), hr = .5,
  eta = log(2) / c(40, 50), S = -5, gamma = 1, T = 36, minfup = 12
) |> gsBoundSummary()

[LittleBeannie]

I talked with Keaven and learned that S should be a positive number, since they are intervals. We may have some input checking in Q3 release.