I use the `normal_density`

function provided by stan-users-guide:

```
real normal_density(real x, real xc, real[] theta, real[] x_r, int[] x_i) {
real mu = theta[1];
real sigma = theta[2];
return 1 / (sqrt(2 * pi()) * sigma) * exp(-0.5 * ((x - mu) / sigma)^2);
}
```

And I compute the integrals:

```
print("integral [N -1.5]: ", integrate_1d(normal_density, negative_infinity(), -1.54835, {0.0, 1.0}, x_r, x_i, 1e-8))
print("integral [-1.5 P]: ", integrate_1d(normal_density, -1.54835, positive_infinity(), {0.0, 1.0}, x_r, x_i, 1e-8))
print("integral [N 1.5]: ", integrate_1d(normal_density, negative_infinity(), 1.54835, {0.0, 1.0}, x_r, x_i, 1e-8))
print("integral [1.5 P]: ", integrate_1d(normal_density, 1.54835, positive_infinity(), {0.0, 1.0}, x_r, x_i, 1e-8))
```

The results are:

```
integral [N -1.5]: 0.939231 // this one is confusing
integral [-1.5 P]: 0.939231
integral [N 1.5]: 0.939231
integral [1.5 P]: 0.060769
```

The integral of (negative_infinity(), -1.54835) is 0.939231. Is it a bug? I use pystan 2.19.1.1.