fun StringBuilder.takeFirst(): Char {
  if (this.length() == 0) return '\0'
  val c = this.charAt(0)
  this.deleteCharAt(0)
  return c
}

fun foo(expr: StringBuilder): Int {
  val c = expr.takeFirst()
  when(c) {
    '\0' => throw new Exception("zero")
    else => throw new Exception("nonzero" + c)
  }
}