 int [1:100]  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15, 16...
 @ x: chr "a string"
 @ y: int [1:20] 1 2 3 4 5 6 7 8 9 10 ...
 @ z:'data.frame':	3 obs. of  1 variable:
  ..$ x: int [1:3] 1 2 3
