Position

Every position value occurs at in the collection.

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Position(collection, value)every position value occurs at, each wrapped in its own

List

Typed signatures

Type	Package	Applies
`(indexed_collection<T>, predicate: (T) any -> boolean) -> list<integer> where T: any`	`compute-engine`	always
`(indexed_collection<any>, any) -> list<integer> \| list<list<integer>>`	`combinatorics`replaces `compute-engine`	always

Domain: Collections

Details

Wolfram's answer where IndexOf reports only the first occurrence, as a plain index.
An empty $List$ when the value isn't present.
Each position is a $List$ of indices, one per level: the search goes into nested collections, so $\{2, 3\}$ is the third part of the second.
Heads count as well, at index $0$ : Position({{1}, {2}}, List) is {{0}, {1, 0}, {2, 0}}.