The environment in which the evaluation
is done (or inherits from if local is TRUE).
substitute
If TRUE, argument expr is
substitute():ed, otherwise not.
globals
(optional) a named list of global objects needed in order
for the future to be resolved correctly.
packages
(optional) a character vector specifying packages
to be attached in the R environment evaluating the future.
local
If TRUE, the expression is evaluated such that
all assignments are done to local temporary environment, otherwise
the assignments are done to the global environment of the R process
evaluating the future.