Specifically, we define that an action a t at time step t is to (1) select a term x 1 from the remaining vocabulary V t ; (2) remove x 1 from V t , and (3) attach x 1 as a hyponym of one term x 2 that is already on the current taxonomy T t .
the size of action space at time step t is |V t | × |T t |, where |V t | is the size of the remaining vocabulary V t , and |T t | is the number of terms on the current taxonomy.