Returns a new preprocessor object which writes the processed text to out. includes should be a list of directories to be searched when files are included.
Reads all input from the file filename, processes the text using proc and writes the output to proc:out. If strip is :strip, then empty lines in stream are not written to proc:out.
Reads all input from stream, processes the text using proc and writes the output to proc:out. If strip is :strip, then empty lines in stream are not written to proc:out.