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`asSeenFrom` produced a typeref of the shape `T'#A` where `A` referred to a symbol
defined in a `T` of times past.
More precisely, the `TypeRef` case of `TypeMap`'s `mapOver` correctly modified a prefix
from having an underlying type of `{ type A = AIn }` to `{ type A = Int }`,
with a new symbol for `A` (now with info `Int`), but the symbol in the outer
`TypeRef` wasn't co-evolved (so it still referred to the `A` in `{ type A = AIn }`
underlying the old prefix).
`coEvolveSym` used to only look at prefixes that were directly `RefinedType`s,
but this bug shows they could also be `SingleType`s with an underlying `RefinedType`.
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