Okay, we can now define a further notion called C command  the phrase X  C commands a phrase Y, if and only if A  neither of X nor Y dominates the other  so neither is  directly above the other and in the path  and B, the first branching node is dominating X, also dominates Y  so let's find out if X and C commands to Y, you go up the tree from X and you find the branching node, the first branching node dominating X and see if that node also dominates  Y.  Okay.