> df <- data.frame(g = c(1, 2, 2, 3, 3, 3), x = 1:6, y = 6:1) %>% group_by(g)
> df %>% do(head, tail)
Error: Can only supply one unnamed argument, not 2.


unnamed elements must return data frames
========================================

> df %>% ungroup() %>% do(1)
Error: Result must be a data frame, not numeric

> df %>% do(1)
Error: Results 1, 2, 3 must be data frames, not numeric

> df %>% do("a")
Error: Results 1, 2, 3 must be data frames, not character


can't use both named and unnamed args
=====================================

> df %>% do(x = 1, 2)
Error: Arguments must either be all named or all unnamed.

