strange function"std_logic_vector"

hi,every one. I found a strange funtion in package "ieee.unsigned_std ";
like following:
----------------------------------------------------------------
library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.std_logic_arith.all;

package STD_LOGIC_UNSIGNED is
.....
function "+"(L: STD_LOGIC_VECTOR; R: STD_LOGIC_VECTOR) return STD_LOGIC_VECTOR is
-- pragma label_applies_to plus
constant length: INTEGER := maximum(L'length, R'length);
variable result : STD_LOGIC_VECTOR (length-1 downto 0);
begin
result := UNSIGNED(L) + UNSIGNED(R);-- pragma label plus
return std_logic_vector(result);
end;
.......
----------------------------------------------------------------
do you notice the line"return std_logic_vector(result);"? "result" is the type of unsigned,and it is converted to the type of std_logic_vector.But, where does the function "std_logic_vector(arg: unsigned)" come from?
I have looked for it in IEEE.std_logic_1164 and IEEE.std_logic_arith,
and did not found it.
If I want to convert unsigned type to std_logic_vector,can I just do it in this way " std_logic_vector(result)", and needn't to use the function of conv_std_logic_vector which is in the package of ieee.std_logic_arth?

guaji