emm is meant to be called only from "glht" as its second
(linfct) argument. It works similarly to multcomp::mcp,
except with specs (and optionally by and contr
arguments) provided as in a call to emmeans.
If the specifications in ... would result in a list (i.e., an
emm_list object), then by default, only the last element of that list
is passed to glht. However, if ... contains a which
argument consisting of integer values, the list elements with those indexes
are selected and combined and passed on to glht. No checking is done
on whether the indexes are valid, and the keyword which must be spelled-out.