Called whenever `path` is changed in the source folder `src`. `dest`
        is the output folder. The default implementation calls `build` after
        determining that the input file is newer than any of the outputs, or
        any of the outputs does not exist.

def changed(self, src, path, dest):
        """Called whenever `path` is changed in the source folder `src`. `dest`
        is the output folder. The default implementation calls `build` after
        determining that the input file is newer than any of the outputs, or
        any of the outputs does not exist."""
        try:
            mtime = os.path.getmtime(os.path.join(src, path))
            self._build(src, path, dest, mtime)
        except EnvironmentError as e:
            logging.error("{0} is inaccessible: {1}".format(
                termcolor.colored(path, "yellow", attrs=["bold"]),
                e.args[0]
            ))