Querying and setting a particular bit in a varying bit value

Hi!

What will be the most efficient way to query the value of a certain bit in
a varying bit field, and what will be the most efficient way to replace
(set/unset) a certain bit in a varying bit field (dynamic length)?

Unfortunately the AND and OR operation require the bits have same length.

Right now the easiest way seems to use substring function, but this
function will use a lot of expensive bit shifting. Is there any better
way?

Thank you.

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                                   S P Arif Sahari Wibowo
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