Element-Relative Sequence Insertion

[Nordlöw]

How do I insert an element (object) relative to another in a sequence
(or list in my case)? Example:

(insert-after 'X '(a b c) 'b)  => '(a b X c)
(insert-before 'X '(a b c) b)  => '(a X b c)

Thanks in advance,
Nordlöw