```python
def index_map(labels: list) -> dict:
    """
    Return a mapping from each label to every position it holds, counted from zero.
    """
    if not labels:
        return {}

    label_to_positions = {}
    for i, label in enumerate(labels):
        if label in label_to_positions:
            label_to_positions[label].append(i)
        else:
            label_to_positions[label] = [i]

    
    return label_to_positions
```