Consider adding a ``drop'' rows operation

[kendonB]

I know that filter can easily achieve this, but it is error prone when there are NA values.

Dropping using filter(!condition) drops NAs but drop(condition) would keep them.

Hope this helps.

[krlmlr]

Something like drop() has been discussed in #741. You could write a tiny helper that achieves this effect.

[hadley]

Duplicate of #741