def [](task_name, scopes=nil)
      task_name = task_name.to_s
      self.lookup(task_name, scopes) or
        enhance_with_matching_rule(task_name) or
        synthesize_file_task(task_name) or
        fail "Don't know how to build task '#{task_name}'"
    end
def [](task_name, scopes=nil)
      task_name = task_name.to_s
      self.lookup(task_name, scopes) or
        enhance_with_matching_rule(task_name) or
        synthesize_file_task(task_name) or
        fail "Don't know how to build task '#{task_name}'"
    end
Find a matching task for +task_name+.