Oracle text phrase error (DRG-50920). Query parsing and SYN function - oracle

Question

I just started working with Oracle Text, already red the docs but I really struggle to find a solution.
Currently using with progressive relaxation but I keep getting the following error:
ORA-29902: error in executing ODCIIndexStart() routine
ORA-20000: Oracle Text error:
DRG-50920: part of phrase not itself a phrase or equivalence
29902. 00000 - "error in executing ODCIIndexStart() routine"
*Cause: The execution of ODCIIndexStart routine caused an error.
*Action: Examine the error messages produced by the indextype code and
take appropriate action.
I have two questions:
How to escape the query in the tag? Maybe {} ? Without escaping it won't work when I type something like COCA COLA 0,5L.
On the other hand, when the query is enclosed in the escape chars ({ }) and I try with "EL K" it thorws the same exception. "EL" is part of a thesaurus.
Code
Table and index creation:
CREATE TABLE "DAVID"."INVENTAR_DEV" (
"ID" VARCHAR2(16 BYTE)
NOT NULL ENABLE,
"NAME" VARCHAR2(255 BYTE)
NOT NULL ENABLE
);
CREATE INDEX "DAVID"."INV_DEV_NAME_IDX" ON
"DAVID"."INVENTAR_DEV" (
"NAME"
)
INDEXTYPE IS "CTXSYS"."CONTEXT" PARAMETERS ( 'DATASTORE CTXSYS.DEFAULT_DATASTORE FILTER CTXSYS.NULL_FILTER LEXER INVENTAR_DEV_LEXER WORDLIST INV_DEV_WORDLIST STOPLIST CTXSYS.EMPTY_STOPLIST'
);
The SELECT which I'm using:
SELECT /*+ FIRST_ROWS(150) */
XMLELEMENT("object",
XMLForest(i.id "id",
i.name "name"
).getStringVal()
FROM david.inv_dev i
WHERE contains(i.name,
'<query>
<textquery grammar="context"> {EL KOS}
<progression>
<seq><rewrite>transform((TOKENS, "FUZZY(SYN({", "}, inv_thes), 70, 10, weight)", " "))</rewrite></seq>
<seq><rewrite>transform((TOKENS, "FUZZY(SYN({", "}, inv_thes), 70, 10, weight)", " AND "))</rewrite></seq>
</progression>
</textquery>
<score datatype="FLOAT" algorithm="DEFAULT"/>
<order>
<orderkey> Score DESC </orderkey>
</order>
</query>', 1) > 0;
Also created my own WORDLIST and LEXER:
BEGIN
ctx_ddl.create_preference('INVENTAR_DEV_LEXER','BASIC_LEXER');
ctx_ddl.set_attribute('INVENTAR_DEV_LEXER', 'numgroup',',');
ctx_ddl.set_attribute('INVENTAR_DEV_LEXER', 'numjoin','.');
ctx_ddl.set_attribute('INVENTAR_DEV_LEXER', 'skipjoins','.-_%:;/,()?!*+');
ctx_ddl.create_preference('INV_DEV_WORDLIST', 'BASIC_WORDLIST');
ctx_ddl.set_attribute('INV_DEV_WORDLIST','FUZZY_MATCH','GENERIC');
ctx_ddl.set_attribute('INV_DEV_WORDLIST','FUZZY_SCORE','70');
ctx_ddl.set_attribute('INV_DEV_WORDLIST','FUZZY_NUMRESULTS','10');
ctx_ddl.set_attribute('INV_DEV_WORDLIST','SUBSTRING_INDEX','FALSE');
ctx_ddl.set_attribute('INV_DEV_WORDLIST','STEMMER','NULL');
ctx_ddl.set_attribute('INV_DEV_WORDLIST','PREFIX_INDEX','TRUE');
ctx_ddl.set_attribute('INV_DEV_WORDLIST','PREFIX_MIN_LENGTH',3);
ctx_ddl.set_attribute('INV_DEV_WORDLIST','PREFIX_MAX_LENGTH',7);
Ctx_thes.create_thesaurus('inv_thes', FALSE); -- NAMEE, CASE-SENSITIVE
CTX_THES.CREATE_RELATION('inv_thes','el','SYN','elektro');
END;
Update
I realized that SYN({something}, thes) doesn't work when there are multiple words devided by spaces.
So there must be an operator between those words.
The query works with the SYN if I remove the following line from the textquery:
<seq><rewrite>transform((TOKENS, "FUZZY(SYN({", "}, inv_thes), 70, 10, weight)", " "))</rewrite></seq>
But I'm still not sure what could be the reason.

Answer 1

My solution to the problem was to use a custom workaround function instead of SYN and a custom query parsing function.
The workaround function for SYN:
FUNCTION f_synonyms(p_word IN VARCHAR2)
RETURN VARCHAR2
AS
CURSOR c_synonyms (p_word IN VARCHAR2)
IS
SELECT REPLACE(CTX_THES.SYN(p_word, g_thesaurus_name), '|','=')
FROM SYS.dual;
v_retVal VARCHAR(255);
BEGIN
OPEN c_synonyms(p_word);
FETCH c_synonyms INTO v_retVal;
CLOSE c_synonyms;
RETURN v_retVal;
END;