def cross(rows, cols):
    """Given two strings, a and b, concatenate letter in string a
    with letter in string b to create a label for each box in the
    Soduku puzzel grid.

    Args:
        rows - str: alphabet label for box (i.e. side label) for a given row
        cols - str: numeric label for box (i.e. top label) for a given column

    Returns:
        list of all possible comibnations of letters from string a and string b
    """
    return [row_label+col_label for row_label in rows for col_label in cols]