The Expected-but-Missing in Context Retrieval

It is a bit of a puzzle how to look back over past word exchanges between a chatbot and its client (the context), to find the meaning of an indeterminate word in the current input, like "it" or "both". It turns out to be reasonably simple in the case of an incomplete input pattern with a missing but expected part that is referenced by "it" or "both". You simply go back over context and filter out anything except words of the expected but missing type. Then retrieve as many as are called for by the indeterminate word, if they are available in that filtered result.