Below are the abstract, introduction, and conclusion of a computer science research paper. Please summarize the main contribution of the work in a single sentence. Your response should include the summary and no additional text.

Paper text:

This paper extends the proof of density of neural networks in the space of continuous (or even measurable) functions on Euclidean spaces to functions on compact sets of probability measures.

By doing so the work parallels a more then a decade old results on mean-map embedding of probability measures in reproducing kernel Hilbert spaces.  

The work has wide practical consequences for multi-instance learning, where it theoretically justifies some recently proposed constructions.

The result is then extended to Cartesian products, yielding universal approximation theorem for tree-structured domains, which naturally occur in data-exchange formats like JSON, XML, YAML, AVRO, and ProtoBuffer.
This has important practical implications, as it enables to automatically create an architecture of neural networks for processing structured data (AutoML paradigms), as demonstrated by an accompanied library for JSON format.
This work has been motivated by recently proposed solutions to multi-instance learning BID28 ; BID19 ; BID5 and by mean-map embedding of probability measures BID23 .
It generalizes the universal approximation theorem of neural networks to compact sets of probability measures over compact subsets of Euclidean spaces.
Therefore, it can be seen as an adaptation of the mean-map framework to the world of neural networks, which is important for comparing probability measures and for multi-instance learning, and it proves the soundness of the constructions of BID19 ; BID5 .The
universal approximation theorem is extended to inputs with a tree schema (structure) which, being the basis of many data exchange formats like JSON, XML, ProtoBuffer, Avro, etc., are nowadays ubiquitous. This
theoretically justifies applications of (MIL) neural networks in this setting.As the presented proof relies on the Stone-Weierstrass theorem, it restricts non-linear functions in neural networks to be continuous in all but the last non-linear layer. Although
this does not have an impact on practical applications (all commonly use nonlinear functions within neural networks are continuous) it would be interesting to generalize the result to non-continuous non-linearities, as has been done for feed-forward neural networks in BID14 .
This paper extends the proof of density of neural networks in the space of continuous (or even measurable) functions on Euclidean spaces to functions on compact sets of probability measures.