Create String Function With Range Of Numbers

Is it possible to create a string with a range of numbers

1
3
4
5
6
8
10

Result: 1,3:6,8,10

Or

1
2
3
4

Result: 1:4

Or

1
3
5
7

Result: 1,3,5,7

Where consecutive numbers are separated by a ":" and other numbers by a ",". I use the following function to create the ","-concatenate, but I don't know how to create the ":"-ranges.

Function AConcat(a As Variant, Optional Sep As String = "") As String
'By Harlan Grove, March 2002
Dim Y As Variant
If TypeOf a Is Range Then
For Each Y In a.Cells

[Code] .........