def add(a, b):
    """Add two numbers."""
    return a + b


def subtract(a, b):
    """Subtract b from a."""
    return a - b

def multiply(a, b):
    """Multiply two numbers."""
    return a * b

def divide(a, b):
    """Divide a by b."""
    if b == 0:
        raise ValueError("Cannot divide by zero.")
    return a / b