If error, warning, message, stdout, or
stderr are NULL, then no side effects of particular
kinds are included in the output.
The semantics is solely defined by the sides_comparer.
E by default uses sides_similar
(see its description therein), although you are free to override
it manually or via a global option.
# the desired outcome is c(1L, 2L, 3L):P(1:3)
# expecting c(1L, 2L, 3L), with a warning:P(1:3, warning=TRUE)
# note, however, that it is the sides_comparer that defines the semantics