code: public static double logSubtract(double x, double y) {
          if (FastMath.useLogAddTable) {
              return SmoothedLogAddTable.logSubtract(x,y);
          } else {
              return FastMath.logSubtractExact(x,y);
          }
      }
docs: Subtracts two probabilities that are stored as log probabilities.
  Note that x >= y.
  
  @param x log(p)
  @param y log(q)
  @return log(p - q) = log(exp(x) - exp(y))
  @throws IllegalStateException if x < y