def separate_labelled(sequence, unlabelled_aa):
    """Get the sequence of unlabelled amino acids from a sequence.

    Parameters
    ----------
    sequence : str
        String of amino acids.
    unlabelled_aa : container object
        Container (list, string...) of unlabelled amino acids.

    Returns
    -------
    tuple(str, str)
        | The sequences as a tuple of string with:
        |    - the sequence without the unlabelled amino acids
        |    - the unlabelled amino acids in the sequence

    """
    labelled_seq = []
    unlabelled_seq = []
    for char in sequence:
        if char in unlabelled_aa:
            unlabelled_seq.append(char)
        else:
            labelled_seq.append(char)
    return ''.join(labelled_seq), ''.join(unlabelled_seq)
