```python
def steps_on(first: int, count: int) -> list:
    """
    Returns a list of numbers starting from 'first' up to 'count' elements.
    
    :param first: The starting number of the sequence.
    :param count: The number of elements in the sequence.
    :return: A list of numbers from 'first' to 'first + count - 1'.
    """
    if count <= 0:
        raise ValueError("Count must be greater than zero.")
    return list(range(first, first + count))

```

```python
def hole_find(seen: list, first: int, count: int) -> int:
    """
    Finds the first missing number in the sequence from 'first' to 'first + count - 1'.
    
    :param seen: A list of numbers that are present in the sequence.
    :param first: The starting number of the sequence.
    :param count: The number of elements in the sequence.
    :return: The first missing number in the sequence, or None if no number is missing.
    """
    if count <= 0:
        raise ValueError("Count must be greater than zero.")
    
    expected_numbers = set(range(first, first + count))
    missing_numbers = expected_numbers - set(seen)
    
    if missing_numbers:
        return min(missing_numbers)
    else:
        return None
```