As a QL author, I can use pattern matching

Summary: QL is a pure functional programming language. Every functional programming language winds up needing pattern-matching. So we should at least begin to design for this, on the theory that we will need it eventually.

This is a solution in search of a problem, which is why it is only medium-priority for now. But that will probably change.

The syntax is not at all obvious, but note how Haskell does it: pattern-matching is implemented as the signature implementation, and each match is its own body. That's kinda elegant, and possibly very Querkish: we might have a List of implementations, following Haskell's syntax for defining a pattern-matching function. If so, it implies that a parameter for a function implementation specifies either a Type or a concrete value. (Or a typeclass, or, in the long run, possibly a polymorphic type variable.)