attempt_rule

# File lib/rake/task_manager.rb, line 245
    def attempt_rule(task_name, extensions, block, level)
      sources = make_sources(task_name, extensions)
      prereqs = sources.collect { |source|
        trace_rule level, "Attempting Rule #{task_name} => #{source}"
        if File.exist?(source) || Rake::Task.task_defined?(source)
          trace_rule level, "(#{task_name} => #{source} ... EXIST)"
          source
        elsif parent = enhance_with_matching_rule(source, level+1)
          trace_rule level, "(#{task_name} => #{source} ... ENHANCE)"
          parent.name
        else
          trace_rule level, "(#{task_name} => #{source} ... FAIL)"
          return nil
        end
      }
      task = FileTask.define_task({task_name => prereqs}, &block)
      task.sources = prereqs
      task
    end