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intersection 

        # Array.intersection

(from ruby core)
---
    array.intersection(*other_arrays) -> new_array

---

Returns a new Array containing each element found both in `self` and in
all of the given Arrays `other_arrays`; duplicates are omitted; items
are compared using `eql?`:
    [0, 1, 2, 3].intersection([0, 1, 2], [0, 1, 3]) # => [0, 1]
    [0, 0, 1, 1, 2, 3].intersection([0, 1, 2], [0, 1, 3]) # => [0, 1]

Preserves order from `self`:
    [0, 1, 2].intersection([2, 1, 0]) # => [0, 1, 2]

Returns a copy of `self` if no arguments given.

Related: Array#&.

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