Implement Document Predicate APIs

Design a simple document service that stores text documents identified by filename. Implement these APIs: - `InsertDoc(filename, content)`: store or overwrite a document. - `CheckContains(filename, predicate) -> bool`: return whether the document satisfies a boolean keyword expression. Rules for `predicate`: - A term is a word such as `a`, `b`, or `error`. - `term` is true if that word appears anywhere in the document. - Operators supported: `&&` and `||`. - `&&` has higher precedence than `||`. - You may also support parentheses for grouping. Examples: - `a || b || c` is true if the document contains at least one of `a`, `b`, or `c`. - `a && b || c` is true if the document contains both `a` and `b`, or contains `c`. Follow-up 1: - Implement `GetAllFiles(predicate) -> List[filename]`, which returns all documents matching the predicate. Follow-up 2: - Explain how you would scale this service in a distributed system, including a reasonable sharding or partitioning strategy and a replication policy. Assume you are starting from a blank editor or whiteboard and should define any necessary data structures and test cases.