This class can be used to represent literal values. 
 
void setValue(const std::string &v)
It sets the literal value. 
 
const std::string & getValue() const 
It returns the literal value. 
 
Expression * clone() const 
It returns a clone of this object. 
 
This class can be used to represent literal values. 
 
This is an abstract class that models a Filter Encoding expression. 
 
Literal(const std::string &value)
It initializes a new Literal.