// StringBuilderBox: simple buffered concatenation
static box StringBuilderBox {
  birth() { me._buf = [] }
  append(s) { me._buf.push(s)  return me }
  toString() {
    // join by empty string
    local n = me._buf.size()
    if n == 0 { return "" }
    local out = me._buf.get(0)
    local i = 1
    loop (i < n) { out = out + me._buf.get(i)  i = i + 1 }
    return out
  }
}