def u_global(name, prefix='u_', suffix=''):
    """Returns the uncertainty corresponding to a column. For example, the
    column ('roi_atoms', 'Fz', '', '') has uncertainties in the column
    ('roi_atoms', 'u_Fz', '', '')
    """
    if type(name) == tuple:
        i = len(name)-1
        while not name[i]:
            i -= 1
        t = name[:i]
        t += (prefix + name[i] + suffix,)
        t += name[i+1:]
        return t
    elif type(name) == str:
        return prefix + name + suffix
    else:
        return None