आप्,सुप् after अव्यय undergo लोप. आप् is the context of fem. suffixes: टाप्, चाप्, डाप्. For example, in तत्र शालायाम्, शालायाम् is the object specified by the adjective तत्र - which should be in the same विभक्ति. However, तत्र being अव्यय, टाप् is लुक्.