vhdl - Modelsim 错误“不是运算符符号”

Question

我写了代码，但 ModelSim 说：

“unsigned2hexstring”不是运算符符号。

我应该改变什么以及如何使用我的库之类的包？会不会喜欢：图书馆ieee；使用 ieee.std_logic_1164.all；使用 work.prosoft_std.all ???

library ieee; 
use ieee.std_logic_1164.all;

package prosoft_std is 
    constant CopyRigthNotice : string := "Copyright 2016 Prosoft. All rights reserved."; 

    type UNSIGNED is array (NATURAL range <>) of STD_LOGIC;
    type SIGNED is array (NATURAL range <>) of STD_LOGIC;
    function "unsigned2hexString" (ARG: UNSIGNED) return SIGNED;

end prosoft_std; 

package body prosoft_std is 

    function "unsigned2hexString" (ARG : UNSIGNED) return UNSIGNED is 
    variable lengthVect : integer; 
    variable useThatVect : UNSIGNED:= ARG; 
    begin 
        lengthVect := ARG'length rem 4; 
        if (lengthVect != 0) then 
            for i in 0 to lengthVect loop 
                useThatVect := '0' &  useThatVect; 
            end loop; 
        end if; 
    return useThatVect; 
    end "unsigned2hexString"; 

end prosoft_std;