Passed to the function specified with the Extended output_kind

* If Print f, the whole input is fed as strings through f, with expected lines reindented (with spaces). * If Numeric f, the indentation values (i.e. total number of leading spaces) for each lines on which in_lines is true are passed through the function. * If Extended f, every element is fed to f with arguments state element. There is at least an element for each token, but there may be more (whitespace, multiline tokens...). You may safely raise an exception from f to stop further processing. This version can be used for syntax highlighting or storing checkpoints.