vice.dataframe.keys
===================

Returns the keys to the dataframe in their lower-case format

**Signature**: x.keys()

Parameters
----------
x : ``dataframe``
	An instance of this class

Returns
-------
keys : ``list``
	A list of lower-case strings which can be used to access the
	values stored in this dataframe.

Example Code
------------
>>> import vice
>>> example = vice.dataframe({
	"a": [1, 2, 3],
	"b": [4, 5, 6],
	"c": [7, 8, 9]})
>>> example
vice.dataframe{
	a --------------> [1, 2, 3]
	b --------------> [4, 5, 6]
	c --------------> [7, 8, 9]
}
>>> example.keys()
['a', 'b', 'c']