Merging two arrays in a uniform order

[aberba via Digitalmars-d-learn]

Unlike array1 + array2, how can i merge arrays such that:
[a1, a1, a2, a1, a1, a2, a1] //uniform order

where a1 = child of array1,
a2 = child of array2

using a built-in function/algorithm (is/are there anything(s) in 
Phobos for this?). No manual approach.