The mixing vat in a factory receives liquid ingredients through six separate value--- labeled R.S.T.U,Y and Z -- each of which has exactly two settings: open and closed.The mixing vat operator must ensure that each valve is set open or closed according to the following conditions: If T is open, both S and Z must be closed, R and Z cannot both be closed at the same time; If Y is closed, Z must also be closed;S and U cannot both be open at the same time. Which of the following, if given to the mixing vat operator as an instruction, would not determine the setting of any other valve ?