Combining two streams in Bacon.js, but only updating when one stream updates

gistfile1.md

Say I have two properties, a and b. If I want a stream that combines their values, I can use a.combine(b, <fn>). Abstractly, this gets me the following:

a:      0    1              2
b:      0         1    2         3
a,b:    0,0  1,0  1,1  1,2  2,2  2,3

But what I need is a stream that only updates when a updates, but uses the most up-to-date value of b. That is:

a:      0    1              2
b:      0         1    2         3
a,b:    0,0  1,0            2,2

Furthermore, successive values of a may be equal, and I don't want to skip duplicates, so I want:

a:      0    1              1
b:      0         1    2         3
a,b:    0,0  1,0            1,2

How would I build this?

@raimohanska Perfect, thank you.