Covariate search issue, it seems for backward search it has to look for "fit" object

[snailittle]

### Noticed the following error with the example model:

library(nlmixr2)
library(rxode2)
library(vpc)
library(xpose)
library(xpose.nlmixr2)
library(ggplot2)

pheno <- function() {
ini({
tcl <- log(0.008) # typical value of clearance
tv <- log(0.6) # typical value of volume
## var(eta.cl)
eta.cl + eta.v ~ c(1,
0.01, 1) ## cov(eta.cl, eta.v), var(eta.v)
# interindividual variability on clearance and volume
prop.err <- 0.1 # residual variability
})
model({
cl <- exp(tcl + eta.cl) # individual value of clearance
v <- exp(tv + eta.v) # individual value of volume
ke <- cl / v # elimination rate constant
d/dt(A1) = - ke * A1 # model differential equation
cp = A1 / v # concentration in plasma
cp ~ prop(prop.err) # define error model
})
}

fit_saem <- nlmixr(pheno, pheno_sd, "saem",
control=list(print=0),
table=list(cwres=TRUE, npde=TRUE))

rxode2::.rxWithWd(tempdir(), {# with temporary directory

auto3 <- covarSearchAuto(fit_saem, varsVec = c("v", "cl"), covarsVec = c("WT"),
catvarsVec = c("APGR"), restart = TRUE,
searchType = "backward", pVal=list(fwd=0.01,bck=0.001))
})
Error in inherits(fit, "nlmixr2FitCore") : object 'fit' not found

**However When I rename fit_saem to fit it seems to bypass this error. Is it a possible bug? This however ran into another error of "matrix multiplication: problem with matrix inverse;" So it seems whenever a model failed (probably due to over parameterization, an inherent process with most of covariate search models), the covariate search can not go on? **

fit=fit_saem
rxode2::.rxWithWd(tempdir(), {# with temporary directory

auto3 <- covarSearchAuto(fit, varsVec = c("v", "cl"), covarsVec = c("WT"),
catvarsVec = c("APGR"), restart = TRUE,
searchType = "backward", pVal=list(fwd=0.01,bck=0.001))
})

── starting backward search... ────────────────────────────────────────
ℹ use control from pipeline
→ loading into symengine environment...
→ pruning branches (if/else) of saem model...
✔ done
→ finding duplicate expressions in saem model...
[====|====|====|====|====|====|====|====|====|====] 0:00:00

✔ done
Error in .model$saem_mod(.cfg) :
matrix multiplication: problem with matrix inverse; suggest to use solve() instead
Error: matrix multiplication: problem with matrix inverse; suggest to use solve() instead

[mattfidler]

Looks like a couple of issues here.

I will transfer the large one to nlmixr2extra where the covariate search is found.

[mattfidler]

Thanks for the issue, I will look at it when I get a moment.