Question on Debugging a code line

[subhabangalore at gmail.com]

Dear Room,

I was trying to go through a code given in http://en.wikipedia.org/wiki/Forward%E2%80%93backward_algorithm[ Forward Backward is an algorithm of Machine Learning-I am not talking on that
I am just trying to figure out a query on its Python coding.]

I came across the following codes.

>>> states = ('Healthy', 'Fever')
>>> end_state = 'E'
>>> observations = ('normal', 'cold', 'dizzy')
>>> start_probability = {'Healthy': 0.6, 'Fever': 0.4}
>>> transition_probability = {
   'Healthy' : {'Healthy': 0.69, 'Fever': 0.3, 'E': 0.01},
   'Fever' : {'Healthy': 0.4, 'Fever': 0.59, 'E': 0.01},
   }
>>> emission_probability = {
   'Healthy' : {'normal': 0.5, 'cold': 0.4, 'dizzy': 0.1},
   'Fever' : {'normal': 0.1, 'cold': 0.3, 'dizzy': 0.6},
   }

def fwd_bkw(x, states, a_0, a, e, end_st):
    L = len(x)
    fwd = []
    f_prev = {} #THE PROBLEM 
    # forward part of the algorithm
    for i, x_i in enumerate(x):
        f_curr = {}
        for st in states:
            if i == 0:
                # base case for the forward part
                prev_f_sum = a_0[st]
            else:
                prev_f_sum = sum(f_prev[k]*a[k][st] for k in states) ##
 
            f_curr[st] = e[st][x_i] * prev_f_sum
 
        fwd.append(f_curr)
        f_prev = f_curr
 
    p_fwd = sum(f_curr[k]*a[k][end_st] for k in states)

As this value was being called in prev_f_sum = sum(f_prev[k]*a[k][st] for k in states marked ## 
I wanted to know what values it is generating.
So, I had made the following experiment, after 
for i, x_i in enumerate(x): 
I had put print f_prev 
but I am not getting how f_prev is getting the values.

Here, 
x=observations,
states= states,
a_0=start_probability,
a= transition_probability,
e=emission_probability,
end_st= end_state

Am I missing any minor aspect?
Code is running fine. 

If any one of the esteemed members may kindly guide me.

Regards,
Subhabrata Banerjee.