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clojure.core/seq
([coll])
  Returns a seq on the collection. If the collection is
    empty, returns nil.  (seq nil) returns nil. seq also works on
    Strings, native Java arrays (of reference types) and any objects
    that implement Iterable.
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clojure.core/seq?
([x])
  Return true if x implements ISeq
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Obviamente vazio? é baseado em seq. qual é a diferença entre empty? e nada? Estou muuuuito confus

clojure.core/empty?
([coll])
  Returns true if coll has no items - same as (not (seq coll)).
  Please use the idiom (seq x) rather than (not (empty? x))

E mais

(not (seq? ())) ;;false
(not (seq ())) ;;true
(not nil) ;;true