Python and Combinatorics

[Nic]

Hello,
I've a problem in defining a good Python code useful to articulate the 
following algorithm.
Can you help  me please?
Thanks a bunch,
Nic

1. Insert a number "n".
Example: 3

2. List all the numbers < or = to n.
Example: 1,2,3.

3. Combine the listed numbers each other.
Example:
12
13
23

4. For each combination associate two letters a and b.
Example:
12a
12b
13a
13b
23a
23b

5. Combine the new combinations each other, provided that combinations 
including the same couple of numbers (e.g. 12a and 12b) cannot be combined.
Example:
12a 13a 23a
12a 13b 23a
12a 13b 23b
12b 13a 23a
12b 13b 23a
12b 13b 23b

PS
12a 13a 23a and13a 23a 12a are the same thing.