doc = function (...) {
    doc = new.env(parent = emptyenv())
    doc$words = lapply(list(...), add_node, doc = doc)
    doc
}

par = function (...) {
    structure(list(...), class = c("par", "docnode"))
}

sentence = function (words) {
    structure(words, class = c("sentence", "docnode"))
}

# Note: for this simple example the `doc` argument to `add_node` is unused but
# for a real document API that allows nessted structures it becomes
# indispensable.
add_node = function (node, doc) {
    UseMethod('add_node')
}

add_node.par = function (node, doc) {
    lapply(node, add_node, doc = doc)
}

add_node.sentence = function (node, doc) {
    unclass(node)
}