dataframe_subset_impl
=====================

A dataframe whose value is a (row-wise) subset of another dataframes, in the 
sense of the "subset" command of "R".  The subset will be re-run as needed to 
reflect changes in the  source dataframe; evaluation is lazy. The subset will
be `subset(eN["x_dataframe"],eval(parse(key_col))%in%key)`.  Is a setting_user.

set_df
======

	some_dfm set_df x_dataframe

Indicate that the x argument of the R "subset" command should be `eN["x_dataframe"]`.

Setting name: x

set_select_from
===============

	some_dfn set_select_from key_col

Indicate that `key_col` is the column whose values should be consulted to determine
which rows to include. Quotation marks are required if an R variable is not desired.

Setting name: x_column

set_values
==========

	some_dfn set_values key...

Indicate that rows that have the value `key` in the key column are those to be 
included.  c() syntax around quotation-mark-enclosed strings, or a 
single quotation-mark-enclosed string is normally required.

Setting name: key