dataframe_columns_impl
=====================

A dataframe whose value is a (column-wise) subset of another dataframes. 
The subset will be re-run as needed to reflect changes in the source
dataframe; evaluation is lazy.  The subset will be
`eN["x_dataframe"][cols]`.  Is a setting_user.

set_df
======

	some_dfm set_df x_dataframe

Indicate that the source dataframe should be `eN["x_dataframe"]`.

Setting name: x

set_columns
===========

	some_dfn set_columns cols...

Indicate that `cols` are the required columns.  Needs quotations marks 
around item(s); multiple items should be in `c()` format.

Setting name: x_column