R Under development (unstable) (2024-06-05 r86695 ucrt) -- "Unsuffered Consequences"
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> library(nlme)
> fm1 <- lme(distance ~ age, Orthodont)
> str(o1 <- getData(fm1))
Classes 'nfnGroupedData', 'nfGroupedData', 'groupedData' and 'data.frame':	108 obs. of  4 variables:
 $ distance: num  26 25 29 31 21.5 22.5 23 26.5 23 22.5 ...
 $ age     : num  8 10 12 14 8 10 12 14 8 10 ...
 $ Subject : Ord.factor w/ 27 levels "M16"<"M05"<"M02"<..: 15 15 15 15 3 3 3 3 7 7 ...
 $ Sex     : Factor w/ 2 levels "Male","Female": 1 1 1 1 1 1 1 1 1 1 ...
 - attr(*, "outer")=Class 'formula'  language ~Sex
  .. ..- attr(*, ".Environment")=<environment: R_GlobalEnv> 
 - attr(*, "formula")=Class 'formula'  language distance ~ age | Subject
  .. ..- attr(*, ".Environment")=<environment: R_GlobalEnv> 
 - attr(*, "labels")=List of 2
  ..$ x: chr "Age"
  ..$ y: chr "Distance from pituitary to pterygomaxillary fissure"
 - attr(*, "units")=List of 2
  ..$ x: chr "(yr)"
  ..$ y: chr "(mm)"
 - attr(*, "FUN")=function (x)  
  ..- attr(*, "source")= chr "function (x) max(x, na.rm = TRUE)"
 - attr(*, "order.groups")= logi TRUE
> 
> df <- Orthodont # note that the name conflicts with df in the stats
> fm2 <- lme(distance ~ age, df)
> str(o2 <- getData(fm2))
Classes 'nfnGroupedData', 'nfGroupedData', 'groupedData' and 'data.frame':	108 obs. of  4 variables:
 $ distance: num  26 25 29 31 21.5 22.5 23 26.5 23 22.5 ...
 $ age     : num  8 10 12 14 8 10 12 14 8 10 ...
 $ Subject : Ord.factor w/ 27 levels "M16"<"M05"<"M02"<..: 15 15 15 15 3 3 3 3 7 7 ...
 $ Sex     : Factor w/ 2 levels "Male","Female": 1 1 1 1 1 1 1 1 1 1 ...
 - attr(*, "outer")=Class 'formula'  language ~Sex
  .. ..- attr(*, ".Environment")=<environment: R_GlobalEnv> 
 - attr(*, "formula")=Class 'formula'  language distance ~ age | Subject
  .. ..- attr(*, ".Environment")=<environment: R_GlobalEnv> 
 - attr(*, "labels")=List of 2
  ..$ x: chr "Age"
  ..$ y: chr "Distance from pituitary to pterygomaxillary fissure"
 - attr(*, "units")=List of 2
  ..$ x: chr "(yr)"
  ..$ y: chr "(mm)"
 - attr(*, "FUN")=function (x)  
  ..- attr(*, "source")= chr "function (x) max(x, na.rm = TRUE)"
 - attr(*, "order.groups")= logi TRUE
> stopifnot(identical(o1, o2))
> 
> proc.time()
   user  system elapsed 
   0.21    0.06    0.28