dev_df   = manifest[manifest[cfg.test_column] != cfg.test_group]
test_df  = manifest[manifest[cfg.test_column] == cfg.test_group]

train_df = dev_df[dev_df[cfg.fold_column] != cfg.val_fold]
val_df   = dev_df[dev_df[cfg.fold_column] == cfg.val_fold]

# no patient may appear in more than one split
for a, b in [(train_df, val_df), (train_df, test_df), (val_df, test_df)]:
    assert not (set(a["Case"]) & set(b["Case"])), "patient leakage"