A "sub" Schema used to select types for a specific use.
This class uses a custom key set to subset a parent Schema, and is used as the return type of {@link SchemaImpl.profile}.
This Schema is not mutable, serving only as a view, you may however define a more specific subset if needed.
Schema is often used to place limitation on expressed content (as in the case of the GML Level 0 Profile), or used to define a non conflicting set of "bindings" for the TypeBuilder(s).
@author Jody Garnett, Refractions Research Inc.
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