# `∩` — Duplicates / Intersection

Function. Keys: `Alt-c`. Ranks: `∞` monadic, `∞ ∞` dyadic

`∩Y` keeps the distinct major cells of `Y` that occur more than once, in
order of first appearance. It matches cells as [`∪`](down-shoe.qmd)
does.

``` bpl
∩"mississippi"      ⍝ "isp"
∩3 1 3 3 2 1        ⍝ 3 1
```

`X∩Y` keeps the major cells of `X` found among the major cells of `Y`.
Order and repetitions are retained. Uses tolerant matching.

``` bpl
1 2 1 3∩1 3        ⍝ 1 1 3
"abcd"∩"bdf"        ⍝ "bd"
[1 2⋄3 4⋄1 2]∩[1 2⋄5 6] ⍝ [1 2⋄1 2]
```

## Errors

- `RANK`: `Y` is a unit
- `LENGTH`: the major cells of `X` and `Y` differ in shape