```python
def cap_after(passage: str) -> str:
    """
    Return a passage with the first letter of every sentence in capitals, a sentence beginning at the start of the passage and again after any full stop. Letters anywhere else are left exactly as they are.
    """
    if not passage:
        return passage

    sentences = passage.split('.')
    capitalized_sentences = [sentence.capitalize() for sentence in sentences]
    capitalized_passage = '. '.join(capitalized_sentences)

    return capitalized_passage
```