Pair

or Juxtapose is a primitive function equivalent to the dfn  which aids in constructing nested arrays during tacit programming where stranding by juxtaposition is not available. It is also equivalent to the catenation of the enclosed arguments, written  using Over. It was introduced in Extended Dyalog APL, and then adopted into dzaima/APL. It was added to BQN with the glyph  in 2021.

Common usage
Its plain usage is in pairing up two parallel values:

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It can also be combined with Mix to increase rank rather than depth:

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Documentation

 * BQN (as )