code: static BIG modSub(BIG a, BIG b, BIG m) {
          return modAdd(a, BIG.modneg(b, m), m);
      }
docs: Modsub takes input BIGs a, b, m and returns a-b modulo m
  
  @param a the minuend of the modular subtraction
  @param b the subtrahend of the modular subtraction
  @param m the modulus
  @return returns a-b (mod m)