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