sues or use non-class dependent variable constraints

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##Row-wise *and* column-wise constraints!

An R-packages that performs row-wise checks: `validate`


```r
library(validate)
```

Note: Different use of the term "validation" - no longer about format, type and range, but used as synonym to "check". 

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##`validate` - overview:

* Splits checking into two steps:
  1. Define the checking rules in a `validator` object, using the `valdiator()` function.
  2. Confront the data with the rules in a call 