code: func ParseExpr(input string) (*Expr, error) {
  	body, err := ParseBody(input)
  	if err != nil {
  		return nil, errors.Wrap(err, "failed to parse expression")
  	}
  	if len(body) != 1 {
  		return nil, fmt.Errorf("expected exactly one expression but got: %v", body)
  	}
  	return body[0], nil
  }
docs: // ParseExpr returns exactly one expression.
  // If multiple expressions are parsed, an error is returned.