A numbering scheme like this?\r\n
\r\n1.a\r\n  2.a\r\n    3.a\r\n      4.a\r\n  2.b\r\n  2.c\r\n    3.a\r\n    3.b\r\n      4.a\r\n
\r\n

Sure, there are 2 3.a's out there. But it is easy to figure out which 2.? a 3.a is actually associated with.

\r\n\r\nCheers,
\r\nBen