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

Attaching package: 'dplyr'

The following objects are masked from 'package:stats':

    filter, lag

The following objects are masked from 'package:base':

    intersect, setdiff, setequal, union


Attaching package: 'igraph'

The following objects are masked from 'package:dplyr':

    as_data_frame, groups, union

The following objects are masked from 'package:stats':

    decompose, spectrum

The following object is masked from 'package:base':

    union

--- finished re-building ‘cran.Rmd’

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

Attaching package: 'igraph'

The following objects are masked from 'package:stats':

    decompose, spectrum

The following object is masked from 'package:base':

    union


Attaching package: 'dplyr'

The following objects are masked from 'package:igraph':

    as_data_frame, groups, union

The following objects are masked from 'package:stats':

    filter, lag

The following objects are masked from 'package:base':

    intersect, setdiff, setequal, union

Quitting from lines 57-63 (degree.Rmd) 
Error: processing vignette 'degree.Rmd' failed with diagnostics:
cannot open the connection
--- failed re-building ‘degree.Rmd’

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

Attaching package: 'dplyr'

The following objects are masked from 'package:stats':

    filter, lag

The following objects are masked from 'package:base':

    intersect, setdiff, setequal, union


Attaching package: 'igraph'

The following objects are masked from 'package:dplyr':

    as_data_frame, groups, union

The following objects are masked from 'package:stats':

    decompose, spectrum

The following object is masked from 'package:base':

    union

Quitting from lines 77-79 (introduction.Rmd) 
Error: processing vignette 'introduction.Rmd' failed with diagnostics:
cannot open the connection
--- failed re-building ‘introduction.Rmd’

SUMMARY: processing the following files failed:
  ‘degree.Rmd’ ‘introduction.Rmd’

Error: Vignette re-building failed.
Execution halted