dataframe_merge_impl
====================

A dataframe whose value is the merge of two dataframes, in the sense of the "merge"
command of "R".  The merge will be re-run as needed to reflect changes in the 
source dataframes; evaluation is lazy.  

set_x
=====

	some_dfm set_x x_dataframe

Indicate that the x argument of the R "merge" command should be `eN["x_dataframe"]`.
The x component of the suffixes argument will be `x_dataframe`.

set_y
=====

	some_dfm set_y y_dataframe

Indicate that the y argument of the R "merge" command should be `eN["y_dataframe"]`.
The y component of the suffixes argument will be `y_dataframe`.

set_x_key
=========

	some_dfn set_x_key key_col...

Indicate that the by.x argument of the R "merge" command should be `key_col`.
If this is a single column, the quotation marks are required, and if this is 
a multiple column constraint, the c() is necessary.

set_y_key
=========

	some_dfn set_y_key key_col...

Indicate that the by.y argument of the R "merge" command should be `key_col`.
If this is a single column, the quotation marks are required, and if this is 
a multiple column constraint, the c() is necessary.