a character vector listing the types of
dependencies, a subset of
c("Depends", "Imports", "LinkingTo", "Suggests", "Enhances").
Character string "all" is shorthand for that vector, and
"most" currently shorthand for these apart from "Enhances".
recursive
logical: should reverse dependencies of reverse
dependencies (and so on) be included?
lib.loc
a character vector of R library trees, or NULL
for all known trees (see .libPaths).