def add_variable_mod(peps:list, mods_variable_dict:dict)->list:
    """
    Function to add variable modification to a list of peptides.
    Args:
        peps (list): List of peptides.
        mods_variable_dict (dict): Dicitionary with modifications. The key is AA, and value is the modified form (e.g. oxM).
    Returns:
        list : the list of peptide forms for the given peptide.
    """
    peptides = []
    for pep_ in peps:
        pep, min_idx = pep_
        for mod in mods_variable_dict:
            for i in range(len(pep)):
                if i >= min_idx:
                    c = pep[i]
                    if c == mod:
                        peptides.append((pep[:i]+[mods_variable_dict[c]]+pep[i+1:], i))
    return peptides