modelLines: The model lines that are being considered
rxui: The rxode2 UI object
modifyIni: Should the ini() be considered
append: This is a boolean to determine if the lines are appended in piping. The possible values for this is:
TRUE which is when the lines are appended to the model instead of replaced
FALSE when the lines are replaced in the model (default)
NA is when the lines are pre-pended to the model instead of replaced
lhs expression, which will append the lines after the last observed line of the expression lhs
auto: This boolean tells if piping automatically selects the parameters should be characterized as a population parameter, between subject variability, or a covariate. When TRUE this automatic selection occurs. When FALSE this automatic selection is turned off and everything is added as a covariate (which can be promoted to a parameter with the ini statement). By default this is TRUE, but it can be changed by options(rxode2.autoVarPiping=FALSE).
cov: is a character vector of variables that should be assumed to be covariates. This will override automatic promotion to a population parameter estimate (or an eta)