In English, we often drop the subject of a phrase, such as in these signs:
Who are these signs written for? It's really *You, stop* or *You, yield* or *You, be alert for bears* (I'm not doing it).
After internalizing a language, we can take hints without explicit instructions. But, to put it politely, math isn't usually well-internalized.
Let's get clear about who the "math signs" are referring to.