[R] How to compute contrast between more than 2 cells? (in contrast package)

[blue sky]

I have contacted the author of the contrast package, but received no
reply. Could somebody that I have the experience on this package give
me some hint.

In the vignette of the package, I only see example to compute
contrasts between 2 cells. Is there a way to compute the contrast
between more than 2 cells. For example, is there a way to compute the
contrast indicated by beta_3 (on page 4 of the vignette), which is a
linear combination of the 4 cell means. Is there a way to use your
contrast() function to compute this contrast?