Add a few blank lines here, so you have:

```D
// Mutable constructor creates mutable object.
S1 m1 = S1(1);

// Constructed mutable object is implicitly convertible to const.
const S1 c1 = S1(1);

// Constructed mutable object is not implicitly convertible to immutable.
// immutable i1 = S1(1);

// Mutable constructor cannot construct immutable object.
// auto x1 = immutable S1(1);

// Immutable constructor cannot construct mutable object.
// auto x2 = S2(1);

// Constructed immutable object is not implicitly convertible to mutable.
// S2 m2 = immutable S2(1);

// Constructed immutable object is implicitly convertible to const.
const S2 c2 = immutable S2(1);

// Immutable constructor creates immutable object.
immutable i2 = immutable S2(1);
```

Remember that this is **user documentation**, it has to be readable.