to_sql_sexp
Description:
Converts a search expression to an SQL 'WHERE' part statement, without the 'WHERE' keyword.
Parameters:
| sexp | 
           a search expression to convert  | 
      
Returns:
| 
           a newly allocated string, an SQL 'WHERE' part statement, or null, when could not convert it. Free it with g_free, when done with it.  |