from typing import Optional


def comment(c: str, inner: str, units: Optional[str] = None):
    """Add 'Begin c' and 'End c' comments around an inner block.

    Optionally add another 'units' comment before the inner block.

    """
    units_str = "" if units is None else f"# {units}\n"
    return units_str + f"# Begin {c}\n" + inner + f"\n# End {c}"