def get_syst ( syst , *index ) :
    """Helper function to decode the systematic uncertainties
    Systematic could be 
    - just a string
    - an object with index:   obj  [ibin]
    - a kind of function:     func (ibin)
    """
    if   isinstance ( syst , str )   : return syst
    elif syst and hasattr  ( syst , '__getitem__' )  : return str ( syst [  index ] )
    elif syst and callable ( syst )                  : return str ( syst ( *index ) )
    elif syst : raise AttributeError("Invalid systematic %s/%s" % ( syst , type( syst ) ) )
    
    return ''