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export function _concat(set1, set2){  set1 = set1 || []  set2 = set2 || []  let idx  const len1 = set1.length  const len2 = set2.length  const result = []
  idx = 0  while (idx < len1){    result[ result.length ] = set1[ idx ]    idx += 1  }  idx = 0  while (idx < len2){    result[ result.length ] = set2[ idx ]    idx += 1  }
  return result}