</pre>

<p>
the variable <code>s</code> of type <code>Slice</code> must be assignable to
the function parameter type <code>S</code> for the program to be valid.
To reduce complexity, type inference ignores the directionality of assignments,
so the type relationship between <code>Slice</code> and <code>S</code> can be
expressed via the (symmetric) type equation <code>Slice ≡<sub>A</sub> S</code>