// ArrayExtBox: tiny helpers for arrays
static box ArrayExtBox {
  each(arr, fn) {
    local i = 0
    local n = arr.size()
    loop (i < n) { fn(arr.get(i), i)  i = i + 1 }
    return arr
  }
  map(arr, fn) {
    local out = []
    local i = 0
    local n = arr.size()
    loop (i < n) { out.push(fn(arr.get(i)))  i = i + 1 }
    return out
  }
  filter(arr, fn) {
    local out = []
    local i = 0
    local n = arr.size()
    loop (i < n) {
      local v = arr.get(i)
      if fn(v) { out.push(v) }
      i = i + 1
    }
    return out
  }
  join(arr, delim) {
    local n = arr.size()
    if n == 0 { return "" }
    local s = arr.get(0)
    local i = 1
    loop (i < n) { s = s + delim + arr.get(i)  i = i + 1 }
    return s
  }
}