Suppose I start with one pair of rabbit and each pair reproduces 3 pairs of rabbits. Let new rabbit pair be `N`

and mature rabbit pair be `M`

:

```
N 0
M 1
MNNN 4
MNNN MMM MMM MMM 19
```

So the series is `0 1 4 19`

How do we do this on Python using loops:

I tried:

```
x="N" #new rabbit pair
x=x.replace("N","M")
x
'M' #Mature rabbit pair
x=x.replace("M","MNNN")
x
'MNNN'
x=x.replace("M","MN")
x=x.replace("N","M")
x
'MNNNMMM'
```

How do I put this in a loop using xrange/range function.

Thanks