Would it be useful to have an update method for nls? I learned recently that the default method can generate an error unless the rhs of a formula is protected by being wrapped in a function, such as log or I(), when terms.formula is called. https://stat.ethz.ch/pipermail/r-help/2010-January/225822.html https://stat.ethz.ch/pipermail/r-help/2010-January/225837.html Would the following be a useful way to ensure this? update.nls <- function(object, formula, ...){ formula <- if(!missing(formula)){ tmp <- formula[[3]] formula[[3]] <- as.call(list(as.name("I"), as.call(tmp))) formula } NextMethod("update", ...) } Ken -- Ken Knoblauch Inserm U846 Stem-cell and Brain Research Institute Department of Integrative Neurosciences 18 avenue du Doyen L?pine 69500 Bron France tel: +33 (0)4 72 91 34 77 fax: +33 (0)4 72 91 34 61 portable: +33 (0)6 84 10 64 10 http://www.sbri.fr/members/kenneth-knoblauch.html