Including special characters in query generation

I want to generate a script for all users.
 What is wrong here? There is one set parameter to set special characters 
off.
 select 'exec dbms_stats.gather_schema_stats 
(ownname=>''''||username||'''||' 
estimate_percent=>dbms_stats.auto_sample_size,method_opt=>'||'''for all 
columns size repeat''',degree =>7)' from dba_users; 
select 'exec dbms_stats.gather_schema_stats 
(ownname=>'||'''||username||'''||' 
estimate_percent=>dbms_stats.auto_sample_size,method_opt=>'||'''||for all 
columns size repeat||''',degree =>7)' from dba_users;
 can anyone tell me what is missing here ? 
 Regards - Chirag

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