def prefix(expof10):
    '''
    Args:

        expof10 : Exponent of a power of 10 associated with a SI unit
            character.

    Returns:

        str : One of the characters in "yzafpnum kMGTPEZY".
    '''
    prefix_levels = (len(SI_PREFIX_UNITS) - 1) // 2
    si_level = expof10 // 3

    if abs(si_level) > prefix_levels:
        raise ValueError("Exponent out range of available prefixes.")
    return SI_PREFIX_UNITS[si_level + prefix_levels]