)The arrow  formula_91  is the group homomorphism from "FY" into "FGFY" sending each generator "y" of "FY" to the corresponding word of length one ("y") as a generator of "FGFY".The arrow  formula_92  is the group homomorphism from "FGFY" to "FY" sending each generator to the word of "FY" it corresponds to (so this map is "dropping parentheses").