matrix_expansion_keys.rb

          EXPANSION_KEYS_FEATURE = [:os]

          EXPANSION_KEYS_LANGUAGE = {
            'c'           => [:compiler],
            'c++'         => [:compiler],
            'clojure'     => [:lein, :jdk],
            'cpp'         => [:compiler],
            'crystal'     => [:crystal],
            'csharp'      => [:csharp, :mono],
            'd'           => [:d],
            'dart'        => [:dart, :dart_task],
            'elixir'      => [:elixir, :otp_release],
            'erlang'      => [:otp_release],
            'fsharp'      => [:fsharp, :mono],
            'go'          => [:go],
            'groovy'      => [:jdk],
            'haskell'     => [:ghc],
            'haxe'        => [:haxe],
            'java'        => [:jdk],
            'julia'       => [:julia],
            'node_js'     => [:node_js],
            'objective-c' => [:rvm, :gemfile, :xcode_sdk, :xcode_scheme],
            'perl'        => [:perl],
            'perl6'       => [:perl6],
            'php'         => [:php],
            'python'      => [:python],
            'r'           => [:r],
            'ruby'        => [:rvm, :gemfile, :jdk, :ruby],
            'rust'        => [:rust],
            'scala'       => [:scala, :jdk],
            'smalltalk'   => [:smalltalk],
            'visualbasic' => [:visualbasic, :mono]
          }

          EXPANSION_KEYS_UNIVERSAL = [:env, :branch]