Certainly! Let's create a function `tcgen` that generates test cases for the problem "The Chair on Which Difficult People Sit".

The function will:
1. Randomly determine the number of chairs `n` (1 ≤ n ≤ 100).
2. Randomly determine the number of passengers `m` (m ≤ n).
3. Randomly generate `m` characters representing the countries of the passengers, each being 'A', 'B', 'C', or 'D'.

Here's the test case generator:

```python
import random

def tcgen():
    n = random.randint(1, 100)  # Number of chairs
    m = random.randint(0, n)    # Number of passengers, ensuring m ≤ n
    
    passengers = [random.choice(['A', 'B', 'C', 'D']) for _ in range(m)]
    
    # Format the test case as specified
    test_case = f"{n} {m}\n"
    test_case += "\n".join(passengers)
    
    return test_case

# Example of generating a test case
print(tcgen())
```

Explanation:
1. The number of chairs `n` is generated randomly between 1 and 100.
2. The number of passengers `m` is generated randomly between 0 and `n`, ensuring `m` does not exceed `n`.
3. For each passenger, a character randomly chosen from 'A', 'B', 'C', 'D' represents their country.
4. The test case is formatted as a string with the number of chairs and passengers on the first line, followed by each passenger on a new line.

You can call the `tcgen()` function to generate and print a test case. Each call will produce a different test case due to the use of random values. If you need multiple test cases, simply call the function multiple times.