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foo=hello
bar=world

As a further clarification, if the the ConfigProvider provides a value of the form  ${xxx:yyy}, no further interpolation is done to try to find a provider for xxx, for example.  Also, if a provider does not have a value for the corresponding key, the variable will remained unresolved and the final value will still be of the form ${provider:key}.

Here is an example use case:

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