those first(S)=a,b,c places its respective prod rule to its column
i.e a will have entry aSa, b will have entry bS and c has entry c, a intersection b intersection c has phi, so it's LL
Suppose, if we have episilon prod instead of 'c', for epsilon, we add follow of S, i.e a will come to picture and make it not LL(1).