I'm dense, but...somebody 'splain accumarray() please?

I've only relatively recently acquired recent-enough release to have
accumarray(). Despite my efforts the documentation for it seems to
remain totally opaque to me.

So,

a) Anybody know of any more detailed tutorial/description than that of
the online doc's? Mayhaps the old dog could eventually learn the new
trick w/ another bone to chase, and

b) I was trying to build an index array of the vector 1:k, w/ each row
1:n being one greater than that in the preceding row. I brute-forced it
w/ repmat() and cumsum() but seems like accumarray ought to be the tool.
But, all my efforts were totally unsuccessful (and for the most part I
couldn't even precisely decipher why got what did get)...

So, how to build the following, say...

[1 2 3; 2 3 4; 4 5 6; ...]

?

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