One of the new features of XSLT 2.0 are the new sequences in the data model. A sequence is “an ordered collection of zero or more items”. So no surprise here, just like arrays or lists in any other programming language. But sequences are special:

• Sequences are “flat”, they may not contain other sequences.
• An item is equivalent to a singleton sequence containing that item and vice versa.

To a programmer, this may seem very weird. But to a non-programmer this is obvious. F.e. a sequence of one can of beer is equivalent to one can of beer. And a sequence of 2 times a sequence of 3 cans of beer is only recognizable as a sequence of 6 cans of beer. So this is a beautiful and very natural abstraction.

Yet I don't know any other language that has something like it. I've been thinking about implementing it in Loell, using the following syntax:

x    = 1            // a sequence of one item
seq1 = 1, 2
seq2 = 0, seq1, 3   // 0, 1, 2, 3
seq3 = seq1, seq2   // 1, 2, 0, 1, 2, 3
 
// lists will still be there
lst1 = [0, seq1]    // [0, 1, 2]