code: public SDVariable exponential(String name, double lambda, SDVariable shape) {
          validateInteger("exponential random", shape);
          SDVariable ret = f().randomExponential(lambda, shape);
          return updateVariableNameAndReference(ret, name);
      }
docs: Generate a new random SDVariable, where values are randomly sampled according to a exponential distribution:
  P(x) = lambda * exp(-lambda * x)
  
  @param name   Name of the output variable
  @param lambda Must be > 0
  @param shape  Shape of the new variable
  @return new SDVaribale