How to extract elements from NLP Tree?

Question

I am using the NLP package to parse sentences. How can I extract an element from the Tree output that is created? For example I'd like to grab the Noun Phrases (NP) from the example below:

library(NLP)
library(openNLP)

s <- c(
    "Really, I like chocolate because it is good.", 
    "Robots are rather evil and most are devoid of decency"
)
s <- as.String(s)


sent_token_annotator <- Maxent_Sent_Token_Annotator()
word_token_annotator <- Maxent_Word_Token_Annotator()
a2 <- annotate(s, list(sent_token_annotator, word_token_annotator))

parse_annotator <- Parse_Annotator()

p <- parse_annotator(s, a2)
ptexts <- sapply(p$features, `[[`, "parse")
ptexts

ptrees <- lapply(ptexts, Tree_parse)

ptrees

## [[1]]
## (TOP
##   (S
##     (S
##       (S
##         (ADVP (RB Really))
##         (, ,)
##         (NP (PRP I))
##         (VP
##           (VBP like)
##           (NP (NN chocolate))
##           (SBAR (IN because) (S (NP (PRP it)) (VP (VBZ is) (ADJP (JJ good)))))))
##       (. .)
##       (, ,)
##       (NP (NNP Robots))
##       (VP (VBP are) (ADJP (RB rather) (JJ evil))))
##     (CC and)
##     (S (NP (RBS most)) (VP (VBP are) (ADJP (JJ devoid) (PP (IN of) (NP (NN decency))))))))

I'd like to grab pieces from the Tree but can't figure out from the documentation for Tree_parse. Using str indicates it should be easy to do but I can't achieve it.

I'd like it to return something like:

[1] "I"      "Robots"

Or as a list rather than a vector.

This likely requires having openNLPmodels.en installed from: http://datacube.wu.ac.at/src/contrib/

Download and run

install.packages(
    "http://datacube.wu.ac.at/src/contrib/openNLPmodels.en_1.5-1.tar.gz",  
    repos=NULL, 
    type="source"
)
              `

If it's helpful folks can source the Tree directly in using the curl package from my Dropbox:

library(curl)
ptrees <- source(curl("https://dl.dropboxusercontent.com/u/61803503/Errors/tree.R"))[[1]]