def _proc_file_spec(fspec, folder, lines=None):
if fspec is None:<EOL><INDENT>return None<EOL><DEDENT>elif not fspec:<EOL><INDENT>fp = "<STR_LIT>"<EOL><DEDENT>else:<EOL><INDENT>fp = os.path.join(folder, fspec)<EOL>lines and _write(fp, lines)<EOL><DEDENT>return fp<EOL>
Process a file  specification, writing any data necessary.

:param str fspec: name of the kind of specification being processed
:param str folder: path to folder in which to place file if written
:param Iterable[str] lines: collection of lines to write to file
:return str: path to file written; null iff the specification is null,
    empty iff the specification is empty, and path to file written otherwise