[css3-flexbox] flex: auto

In section 7.2 Common Values of 'flex' [1], the first value to be mapped to its equivalent is 'flex: auto'.

The property definition of the flex shorthand in Section 7.1 [2] does not list the auto keyword, however.
Neither does the property index [3].

[1] http://dev.w3.org/csswg/css3-flexbox/#flex-examples
[2] http://dev.w3.org/csswg/css3-flexbox/#flex-property
[3] http://dev.w3.org/csswg/css3-flexbox/#property