def getIncomparable(rs1, rs2):
    """
    Return the set of problem IDs which are only solved by rs1
    :param rs1: result set of the 1st approach
    :param rs2: result set of the 2nd approach
    :return: a set {pid, stim}
    """
    inc = set()
    for pid in rs1.keys():
        r2 = rs2[pid]
        if r2 is None:
            inc.add(pid)
    return inc