Is there any succinct way to say, in a directed graph, "all vertices of the same level at most 2N edges apart" and all vertices of the same level at exactly 2N edges apart" succinctly? I'm trying to think of good function names for siblings/cousins/siblings+cousins when working with a directed graph (tree/dag)

Can anybody tell me what is subsumption rule and how it is unproblematic? I am following principles of program analysis book for annotated type systems but not understanding how to apply this rule for conditional statement i.e if-else block? Sharing image below for my question

For my own understanding, I am solving these questions. But I am not getting how can I solve this