another data frame.
If any of the variables in
x was itself a matrix or a data frame
each of its columns will be made into a separate column of the result.
The result will print looking much like the original; in particular,
the new column labels for matrix variables will be made by pasting
the original variable name,
".", and the column labels of the variable.
This function is primarily support; e.g., see
pairs.data.frame().