list comprehension return a list and sum over in loop

[Jussi Piitulainen]

KK Sasa writes:

> def p(x,t,point,z,obs):    
>     d = x[0]
>     tau = [0]+[x[1:point]] 
>     a = x[point:len(x)]
>     at = sum(i*j for i, j in zip(a, t))
>     nu = [exp(z[k]*(at-d)-sum(tau[k])) for k in xrange(point)]
>     de = sum(nu, axis=0)
>     probability = [nu[k]/de for k in xrange(point)]
>     return probability[obs]

I must be blind, but this looks like computing a whole probability
distribution and then throwing almost all of it away.

Can't this just return nu[obs]/de?

The expression for tau also seems weird to me. Isn't it equivalent to
[0, x[1:point]], a two-element list with the second element a list?
How can sum(tau[k]) work at all then, for any k > 1?