def strip_solution(s):
    """Remove the solution from a code cell

    Parameters
    ----------
    s : a code cell

    This is currently not used anywhere.

    """
    lines = s.split('\n')
    xformed_lines = []
    OUT, IN = True, False
    for line in lines:
        if OUT:
            if line.strip() == '### BEGIN SOLUTION':
                IN, OUT = True, False
            else:
                xformed_lines.append(line)
        else:
            assert IN
            if line.strip() == '### END SOLUTION':
                OUT, IN = True, False
                line = line.replace('### END SOLUTION', 'pass')
                xformed_lines.append(line)
            else:
                continue
    s = '\n'.join(xformed_lines)
    return s
