(or <code>S ≡<sub>A</sub> Slice</code> for that matter),
where the <code><sub>A</sub></code> in <code>≡<sub>A</sub></code>
indicates that the LHS and RHS types must match per assignability rules
(see the section on <a href="#Type_unification">type unification</a> for
details).
Similarly, the type parameter <code>S</code> must satisfy its constraint
<code>~[]E</code>. This can be expressed as <code>S ≡<sub>C</sub> ~[]E</code>