def export(self, nidm_version, export_dir):
        """
        Create prov graph.
        """
        if self.expl_mean_sq_file is None:
            # Create Contrast Explained Mean Square Map as fstat<num>.nii.gz
            # multiplied by sigmasquareds.nii.gz and save it in export_dir
            fstat_img = nib.load(self.stat_file)
            fstat = fstat_img.get_data()

            sigma_sq_img = nib.load(self.sigma_sq_file)
            sigma_sq = sigma_sq_img.get_data()

            expl_mean_sq = nib.Nifti1Image(
                fstat*sigma_sq, fstat_img.get_qform())

            self.filename = ("ContrastExplainedMeanSquareMap" +
                             self.num + ".nii.gz")
            self.expl_mean_sq_file = os.path.join(
                export_dir, self.filename)
            nib.save(expl_mean_sq, self.expl_mean_sq_file)

        self.file = NIDMFile(self.id, self.expl_mean_sq_file,
                             filename=self.filename,
                             sha=self.sha, fmt=self.fmt)

        # Contrast Explained Mean Square Map entity
        path, filename = os.path.split(self.expl_mean_sq_file)
        self.add_attributes((
            (PROV['type'], self.type),
            (NIDM_IN_COORDINATE_SPACE, self.coord_space.id),
            (PROV['label'], self.label)))
Create prov graph.