Sometimes we may want to exit the knitting process early, and completely
ignore the rest of the document. This function provides a mechanism to
terminate knit().
knit_exit(append, fully = TRUE)A character vector to be appended to the results from
knit() so far. By default, this is \end{document} for LaTeX
output, and </body></html> for HTML output, to make the output
document complete. For other types of output, it is an empty string.
Whether to fully exit the knitting process if knit_exit()
is called from a child document. If FALSE, only exit the knitting
process of the child document.
Invisible NULL. An internal signal is set up (as a side
effect) to notify knit() to quit as if it had reached the end of the
document.
# see https://github.com/yihui/knitr-examples/blob/master/096-knit-exit.Rmd