Error(s) in re-building vignettes:
--- re-building ‘classstructure.Rmd’ using rmarkdown
--- finished re-building ‘classstructure.Rmd’

--- re-building ‘introduction.Rmd’ using rmarkdown

You have fewer observations (18) than number of periods (84). Results will likely be unreliable.
Warning: You are extrapolating ending periods
Total of 5 period(s) imputed

You have fewer observations (19) than number of periods (84). Results will likely be unreliable.
Total of 5 period(s) imputed

You have fewer observations (23) than number of periods (84). Results will likely be unreliable.
Warning in lmrob.S(x, y, control = control) :
  S-estimated scale == 0:  Probably exact fit; check your data
Total of 4 period(s) imputed

You have fewer observations (27) than number of periods (84). Results will likely be unreliable.
Warning in lmrob.S(x, y, control = control) :
  S-estimated scale == 0:  Probably exact fit; check your data
Total of 3 period(s) imputed

You have fewer observations (32) than number of periods (84). Results will likely be unreliable.
Warning in lmrob.S(x, y, control = control) :
  S-estimated scale == 0:  Probably exact fit; check your data
Total of 2 period(s) imputed

You have fewer observations (35) than number of periods (84). Results will likely be unreliable.
Warning in lmrob.S(x, y, control = control) :
  S-estimated scale == 0:  Probably exact fit; check your data
Total of 1 period(s) imputed

You have fewer observations (35) than number of periods (84). Results will likely be unreliable.
Warning in lmrob.S(x, y, control = control) :
  S-estimated scale == 0:  Probably exact fit; check your data
Warning: You are extrapolating ending periods
Total of 2 period(s) imputed
Quitting from lines 946-958 (introduction.Rmd) 
Error: processing vignette 'introduction.Rmd' failed with diagnostics:
nrow(labels) == length(null) is not TRUE
--- failed re-building ‘introduction.Rmd’

SUMMARY: processing the following file failed:
  ‘introduction.Rmd’

Error: Vignette re-building failed.
Execution halted