sonic-pi

https://github.com/sonic-pi-net/sonic-pi

C++

Code. Music. Live.

Parslet.subtree

Returns a placeholder for tree transformation patterns that will match 
any kind of subtree. 

  { :expression => subtree(:exp) }

Source | Google | Stack overflow

Edit

git clone [email protected]:sonic-pi-net/sonic-pi.git
cd sonic-pi
open app/server/ruby/vendor/parslet/lib/parslet.rb

Contribute

# Make a new branch
git checkout -b -your-name--update-docs-Parslet-subtree-for-pr

# Commit to git
git add app/server/ruby/vendor/parslet/lib/parslet.rbgit commit -m "better docs for Parslet.subtree"

# Open pull request
gem install hub # on a mac you can `brew install hub`
hub fork
git push <your name> -your-name--update-docs-Parslet-subtree-for-pr
hub pull-request

# Celebrate!