"""Create fact_checker
Generated by Phi-Octopus Eden
2025-11-08 10:54:01.746854
"""

```python
from typing import List, Optional


class FactChecker:
    """
    A simple fact-checking class that verifies if a given statement is true based on predefined facts.
    """

    def __init__(self):
        self.facts = {
            "global_warming": True,
            "moon_lander_scandal": False,
            "dna_double_helix": True
        }

    def check_fact(self, statement: str) -> Optional[bool]:
        """
        Checks if the given statement is a known fact.

        :param statement: The statement to be checked.
        :return: True if the statement is a known true fact, False if it's a known false fact,
                 None if the statement is unknown or not in the database.
        """
        return self.facts.get(statement.lower())

    def add_fact(self, statement: str, is_true: bool):
        """
        Adds a new fact to the fact-checker.

        :param statement: The statement to be added as a fact.
        :param is_true: True if the statement is true, False otherwise.
        """
        self.facts[statement.lower()] = is_true

    def remove_fact(self, statement: str):
        """
        Removes an existing fact from the fact-checker.

        :param statement: The statement to be removed as a fact.
        """
        if statement in self.facts:
            del self.facts[statement]


# Example usage
checker = FactChecker()
print(checker.check_fact("global_warming"))  # Output: True

checker.add_fact("earth_is_flat", False)
print(checker.check_fact("earth_is_flat"))  # Output: False

checker.remove_fact("moon_lander_scandal")
print(checker.check_fact("moon_lander_scandal"))  # Output: None
```