def fixoffset(df, xoff, yoff):
    """Offset correction for small MM chambers.
    Parameters:
    -----------
        df :    Pandas DataFrame.
        xoff :  offset of x position.
        yoff :  offset of y position.
   
    Returns:
    --------
        df :    modified DataFrame.
    """

    df.loc[df.chbid < 4, 'ypos'] += 17
    df.loc[df.chbid < 4, 'xpos'] += 18
    
    return df