concatenate RAW data in PL/SQL with R7.3.2

In PL/SQL there are concatenation operator and function for VARCHAR2 datatype and its subtype -- "||", concat() -- to put string together such as:

       DECLARE
          string1  VARCHAR(10) := 'one';
	  string2  VARCHAR(10) := ' piece';
          string3  VARCHAR(20);
       BEGIN
	  string3 := string1 || string2;
       END

but there is no such such built-in function for RAW datatype.

Is there any techniques available in PL/SQL to concatenate RAW datatype ?

Thanks in advance, Ming Tsao

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