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filter_map 

        # Enumerator::Lazy.filter_map

(from ruby core)
### Implementation from Lazy
---
    lazy.filter_map { |obj| block } -> lazy_enumerator

---

Like Enumerable#filter_map, but chains operation to be lazy-evaluated.

    (1..).lazy.filter_map { |i| i * 2 if i.even? }.first(5)
    #=> [4, 8, 12, 16, 20]

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