class Calculator:
    def add(self, a, b):
        """Return the sum of a and b."""
        return a + b

    def subtract(self, a, b):
        """Return the difference of a and b (a - b)."""
        return a - b

    def multiply(self, a, b):
        """Return the product of a and b."""
        return a * b

    def divide(self, a, b):
        """Return the quotient of a and b (a / b).

        Raises:
            ValueError: If b is zero, as division by zero is not allowed.
        """
        if b == 0:
            raise ValueError("Cannot divide by zero.")
        return a / b
