code: func ArrayOperand(x ast.Value, pos int) (ast.Array, error) {
  	a, ok := x.(ast.Array)
  	if !ok {
  		return nil, NewOperandTypeErr(pos, x, "array")
  	}
  	return a, nil
  }
docs: // ArrayOperand converts x to an array. If the cast fails, a descriptive
  // error is returned.