predicate.rb

test, predicate = *ast

catch_arg = symbol(:predicate_matched)

process(test, input) do |matched_test_node|
  catch_var   = literal('predicate_matched')
  catch_block = send_message('catch', catch_arg).add_block do
    process(predicate, matched_test_node) do
      send_message('throw', catch_arg, literal('true'))
    end
  end

  catch_var.assign(catch_block)
    .followed_by(catch_var.if_true { yield matched_test_node })
end