def dim_col(d: int) -> str:
    """
    Name of an dimension columns.

    Parameters
    ----------
    d
        Dimension number.

    Returns
    -------
    name: str
        Dimension name.

    Example
    -------
    >>> from rle_array.testing import dim_col
    >>> dim_col(1)
    'dim_1'
    """
    return f"dim_{d}"