def get_P_X_past_uncond(past_symbol_counts, number_of_symbols):
    """
    Compute P(X_past), the probability of the past activity using
    the plug-in estimator.
    """
    
    P_X_past_uncond = {}
    for response in [0, 1]:
        for symbol in past_symbol_counts[response]:
            if symbol in P_X_past_uncond:
                P_X_past_uncond[symbol] += past_symbol_counts[response][symbol]
            else:
                P_X_past_uncond[symbol] = past_symbol_counts[response][symbol]
    number_of_symbols_uncond = sum(number_of_symbols)

    for symbol in P_X_past_uncond:
        P_X_past_uncond[symbol] /= number_of_symbols_uncond
    return P_X_past_uncond