Nothing
grback <- function(par, userfn, fbase=NULL, eps=1.0e-7, ...) {
# Backward difference gradient approximation
if (is.null(fbase)) fbase <- userfn(par, ...) # ensure we function value at par
df <- rep(NA, length(par))
teps <- eps * (abs(par) + eps)
for (i in 1:length(par)) {
dx <- par
dx[i] <- dx[i] - teps[i]
df[i] <- (fbase - userfn(dx, ...))/teps[i]
}
df
}
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