compose takes as its argument a string in any datatype,
and returns a Unicode string in its fully normalized form in the same
character set as the input. string can be any of the datatypes
CHAR, VARCHAR2, NCHAR, NVARCHAR2, CLOB, or NCLOB. For example, an
&o& codepoint qualified by an umlaut codepoint will be returned
as the o-umlaut codepoint.