= Solution
A <covariant type parameter> preserves <subtyping>: \b[if $A<:B$, then `PartialFn[T,A] <: PartialFn[T,B]`] when its second parameter is covariant. In <Scala> the class annotation is `+U`. A mapping that produces a narrower output can be used where a broader output is expected. Covariance describes a type relationship; it does not convert stored values or make mutation through a widened reference safe.<Scala 2.12 types specification>[Relations between types: conformance of parameterized types and variance]{ref}
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