code: public static double meanTruncLower(double mu, double sigma, double lowerBound) {
          double alpha = (lowerBound - mu) / sigma;
          double phiAlpha = densityNonTrunc(alpha, 0, 1.0);
          double cPhiAlpha = cumulativeNonTrunc(alpha, 0, 1.0);
          return mu + sigma * phiAlpha / (1.0 - cPhiAlpha);
      }
docs: Returns the mean of the normal distribution truncated to 0 for values of x < lowerBound