code: func (b *builder) arrayLen(fn *Function, elts []ast.Expr) int64 {
  	var max int64 = -1
  	var i int64 = -1
  	for _, e := range elts {
  		if kv, ok := e.(*ast.KeyValueExpr); ok {
  			i = b.expr(fn, kv.Key).(*Const).Int64()
  		} else {
  			i++
  		}
  		if i > max {
  			max = i
  		}
  	}
  	return max + 1
  }
docs: // arrayLen returns the length of the array whose composite literal elements are elts.