def boundary_line(cx, cy, co):
    """
    Check *boundary_line*.
    :raises TypeError: Parameter cx must be a real number.
    :raises TypeError: Parameter cy must be a real number.
    :raises TypeError: Parameter co must be a real number.
    """
    if not isinstance(cx, (int, float)):
        raise TypeError('\n\nParameter cx must be a real number.\n')
    if not isinstance(cy, (int, float)):
        raise TypeError('\n\nParameter cy must be a real number.\n')
    if not isinstance(co, (int, float)):
        raise TypeError('\n\nParameter co must be a real number.\n')
