Implement composition and mixin utilities

Problem Statement

You are building a data transformation pipeline. You need to implement two distinct utilities: a function composer and a JSON-serializable mixin.

1. Function Composer

Implement a function compose_pipeline(*funcs) that returns a new function. This returned function should accept an arbitrary number of arguments and keyword arguments, pass them to the first function in the chain, and pass the result as the sole positional argument to each subsequent function. If the input list is empty, return the identity function.

2. Serializable Mixin

Create a class JsonMixin that enables any class inheriting from it to be serialized to a JSON string.

• Implement a method to_json(self, include_private=False).
• It should return a JSON string of all instance attributes.
• If include_private is False, exclude any attributes starting with an underscore _.
• If the object contains non-serializable data (e.g., a function object or a custom class instance), catch the error and return a string literal: "[Unserializable]".

Examples

# Pipeline example
def add_five(x): return x + 5
def square(x): return x * x

pipeline = compose_pipeline(add_five, square)
print(pipeline(2)) # (2+5)^2 = 49

# Mixin example
class Data(JsonMixin):
    def __init__(self, val, secret):
        self.val = val
        self._secret = secret

d = Data(10, "hidden")
print(d.to_json(include_private=False)) # '{"val": 10}'

Constraints

• The pipeline must handle variable arguments for the first function only.
• Use json.dumps for the final serialization step.
• Handle potential circular references gracefully if possible, or at least ensure the process does not crash indefinitely.