def compute_transitions(bands,in_list,fin_list):
    """
    Compute the (positive) transition energies for the bands (on a single kpoint).
    The `fast` index is associated to the bands in the fin_list list.

    Args:
        bands (list) : list with the energies
        in_list (list) : indexes of the bands used as starting points of the transitions
        fin_list (list) : indexes of the bands used as final points of the transitions

    Returns:
        :py:class:`list` : list with the transition energies for each possible couple
        of (distinct) in and out bands

    """
    transitions = []
    for v in in_list:
        for c in fin_list:
            if c > v:
                transitions.append(bands[c]-bands[v])
    return transitions