@Override
    public final int valueHashCode(Object value) {
        return value.hashCode();
    }
Implements strict stirng equality semantics by returning the 
<code>java.lang.Object</code>-level <code>hashCode</code> of 
the object.
@param value an object returned by <code>createValue</code>
@return the hash code
@see org.relaxng.datatype.Datatype#valueHashCode(java.lang.Object)
src/nu/validator/datatype/AbstractDatatype.java