commalist : commalist ',' expression
                 | expression

def p_x_commalist(self,t):
    """commalist : commalist ',' expression
                 | expression
    """
    if len(t) == 2: t[0] = CommaX([t[1]])
    elif len(t) == 4: t[0] = CommaX(t[1].children+[t[3]])
    else: raise NotImplementedError('unk_len',len(t)) # pragma: no cover