Programs can be represented in various ways in ProgramTransformationTools.

• AbstractSyntaxTree^?
• DirectedAcyclicGraph^?
• Graph
• Objects

Particularly important/problematic is the treatment of variables and other identifiers with restricted scope.

• SymbolTable^?
• VariableRenaming^?
• HigherOrderAbstractSyntax^?
• FreshML

In addition to the internal representation used by a tool, it is important that tools can export such a representation such that it can be manipulated by other tools. ExchangeFormats facilite the exchange of structured representations of programs between tools.