group_by: if using an expression, new column name cannot be "x"

[Mullefa]

For example:

# example in documentation
group_by(mtcars, vsam = vs + am)

# doesn't work with x
group_by(mtcars, x = vs + am)

This is presumably down to group_by and/or regroup having x as their first argument?!

[kevinushey]

Perhaps the argument name should be .data, thereby matching the signature of filter, summarise etc?