adding a sentinel

cardinal concept: adding a sentinel entity to a given entity that has no sentinel.
Case 1. The sentinel is clearly distinct from the given entity, the canary in the coal mine being an iconic example of this. (cf: the topic of ‘sentinel species’.)
Case 2. The sentinel is tightly combined with the given entity, the stink added to natural gas being an iconic example of this. This gives rise to different phraseology, namely, that references to the given entity are via references to the topological closure of the sentinel entity. For example, if A is the natural gas and B is the stink, then “I smell gas.” really means “I smell B – and therefore A is present.” Those ignorant of the engineered safety of tightly combining a sentinel-bearing entity to a sentinel-less entity will, of course, mean what they say literally.
keywords: topology, indirect reference