LaTeX2e's filecontents and filecontents* environments enable a
LaTeX source file to generate external files as it runs through
LaTeX. However, there are two limitations of these
environments: they refuse to overwrite existing files, and they
can only be used in the preamble of a document. The
filecontents package removes these limitations, letting you
overwrite existing files and letting you use
filecontents/filecontents* anywhere.