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export default Queue
function Queue(initial = []) {  let xs = initial.slice()  let index = 0
  return {    get length() {      return xs.length - index    },    remove: (x) => {      const index = xs.indexOf(x)      return index === -1        ? null        : (xs.splice(index, 1), x)    },    push: (x) => (xs.push(x), x),    shift: () => {      const out = xs[index++]
      if (index === xs.length) {        index = 0        xs = []      } else {        xs[index - 1] = undefined      }
      return out    }  }}