Schemes specifying a concrete syntax and associated protocols define each URI. Some object oriented programming languages have a special syntax for specifying class invariants. Such relational models provide declarative methods for specifying data and queries. A further problem is the difficulty of specifying legitimate criteria. Visual Paradigm provides a SysML requirement diagram for specifying and analyzing requirements. DSC allows for declaratively specifying how a software environment should be configured. There are also domain-specific languages with a focus on specifying and checking architectural constraints. Both these classes have constructors that support specifying the character encoding to use. Timeframes are defined for each release by specifying freezing dates. There was no generally supported way of specifying byte-exact file sizes. Words include an objective semantic element and markers specifying the grammatical use of the word. Use cases are a way of specifying required usages of a system. There are several different languages available for specifying an XML schema. The importance of specifying the reference datum may be illustrated by a simple example. The first element is a number specifying filament voltage.