<timed-effect> := (at <time-specifier> <da-effect>)
                  (at <time-specifier> <f-assign-da>)

<da-effect> := (<assign-op> <f-head> <f-exp-da>)
               ...

<f-assign-da> := (<assign-op> <f-head> <f-exp-da>)
                 ...