Currently, clones "ts -> ty" are represented as "ts -> ts[=ty]", where the second type symbol is an alias created on the fly for the desired type. The problem comes when we want to clone a theory that contains such a cloning declaration. Since these aliases are "non-citizens" -- they are not properly declared, they are not put in the known_map, and they are not listed as locally declared symbols of the theory -- cloning of the cloning declaration mistakes them for external symbols and ignores them. Thus, the aliased type goes into the cloned theory as is, which provokes the error. The fix consists in an additional test for non-local type symbols. If such a symbol is an alias for a type that is actually cloneable, we conclude that the symbol is not external, but is a mere proxy for the aliased type, and so we clone the type and create a fresh type alias for it. A proper fix would be to always clone type symbols into types and never introduce the proxy type aliases. This is a more intrusive change, however, so let's start with a bandage.
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