1. compare each to each and one to many

[MashaPtukha]

If you wanted to compare each groups to each other group, for example, and you have several of them, it would be handy to have a way to define the contrasts automatically, not each by hand

[MashaPtukha]

for pairwise: we can just use a function internal to emmeans

library(emmeans)
fit <- lm(response ~ treatment, data = your_data)
em_means <- emmeans(fit, "treatment")
pairwise_contrasts <- contrast(em_means, method = "pairwise")

to be continued