This function calculates text values of nodes from the 
    scope, and checks whether the current text value 
    contains any fragment that matches the query.  
    When the first match is found, the rest of the node-set is ignored the 
    boolean @true@ is returned.  If the node-set ends before any match is 
    found, @false@ is returned.The text_contains() function may be used only 
    in XPath expressions that are arguments of xcontains().  
    This restriction is for optimization purposes.  When Virtuoso executes an 
    SQL statement that uses xcontains(), it performs some 
    sophisticated free-text search, and it applies the XPath expression not to 
    all available documents but only to documents that satisfied the free-text 
    search criterion.  Moreover, the server uses the intermediate free-text 
    data to optimize the search inside a selected document.