Generic sequences in seq.el

[Yuan Fu <casouri@gmail.com>]

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I'm trying to "upgrade" some methods from supporting list to supporting
generic sequence.
However, I couldn't figure out how to change the value of a sequence
destructively.
E.g., if I want to insert a element into a list, I can do something like

    (setf (nthcdr n seq) (cons element (nthcdr n seq)))

to insert element into the nth position in seq

But with sequences, I don't see any generic functions that can do similar
work.
Am I missing something?

BTW, what I'm doing now is define my own generic function, like

    (cl-defgeneric my-insert-at (elt seq)))

The idea is I'll implement this function for the generic sequence
that I'll define in the future. Is that a correct approach?
Other destructive functions that I couldn't find alternatives are delete,
push, pop, etc.

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