require 'ruby2ruby'

irb(main):040:0> proc { 3 * 3}.to_sexp
=> [:iter, [:fcall, :proc], nil, [:call, [:lit, 3], :, [:array, [:lit, 3]]]]
irb(main):041:0> proc { 3 + 3}.to_sexp.to_s
=> "iterfcallproccalllit3+arraylit3"
irb(main):042:0> proc { 3 + 3}.to_sexp.pretty_inspect
=> "[:iter, [:fcall, :proc], nil, [:call, [:lit, 3], :+, [:array, [:lit, 3]]]]\n"

the :+ isn't shown in the colored output for some reason.