What is the . in formula ~. syntax?

Thank you!  I searched in the manual, but I did not see where this is mentioned, I looked under operators
and in some of the formula documentation.
It *is* documented on the help page for formula (and it is not an 
operator, so should not be in the documentation for operators):

      There are two special interpretations of ?.? in a formula.  The
      usual one is in the context of a ?data? argument of model fitting
      functions and means ?all columns not otherwise in the formula?:
      see ?terms.formula?.  In the context of ?update.formula?, *only*,
      it means ?what was previously in this part of the formula?.

In other contexts it is literally '.' (see ?terms.formula).
Brian

On Nov 23, 2012, at 3:15 AM, Michael Weylandt <michael.weylandt at gmail.com> wrote:

On Nov 23, 2012, at 4:26 AM, Brian Feeny <bfeeny at mac.com> wrote:

I know if I have a dataframe with columns y, x1, x2 and I wish to have y as my y value and x1 and x2 as x values I can do:
y ~ x1 + x2

or

y ~.

but can someone explain what . actually is or what its transposed into?
Everything not already stated.

rmw

I searched for this with no success, reading the "formula" manual pages.

Brian

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Brian D. Ripley,                  ripley at stats.ox.ac.uk
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What is the . in formula ~. syntax?

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