def check_dictionary(src2trg: dict) -> dict:
    """
    Check validity of PanLex dictionary:
        - Each source token only has one target token
        - Source and target tokens are strings

    :param src2trg dict: PanLex dictionary
    :rtype dict: validated PanLex dictionary
    """
    out = {}
    for k, v in src2trg.items():
        assert isinstance(k, str)
        assert len(v) == 1
        assert isinstance(v[0], str)
        out[k] = v[0]
    return out