   which indicates the category of the output to the left and the category of its inputs in order within the angled brackets. Call this the categorial index of the expression. Thus the category of sentential negation is  \(\rS\langle\rS\rangle\),  that of conjunction is \(\rS\langle\rS\rS\rangle\),  while that of the epsilon functor ‘\(\isa\)’ is   \(\rS\langle\rN\rN\rangle\),  since it builds a sentence using two names as arguments.