frontation results
* `aggregate()`: Compute percentage pass/fail/na 
* `sort()`: Sort results by problem prevalence
* `values()`: For each observation and each check: `TRUE`/`FALSE`/`NA`
* `barplot()`: Visual overview of check results
* `errors()`: What errors were caught?
* `warnings()`: What warnings were caught?

---

##Warning: modify-by-reference

* `validator` and `confrontation` objects imply use of "modify-by-reference" rather than "copy-on-modify" semantics (standard R)
* Other package that uses "m