Expressions are used to specify values, looping, conditions for emitting optional elements (forcing them to appear), and the condition for emitting a null value. These expressions essentially define the execution logic.
Expression panels of the above types are associated with structure and map elements. Each expression panel forms a tree where each expression is evaluated in terms of its subordinate expressions. Each tree node in the expression panel is an expression.
Expressions may be presented as tree nodes (as shown in the examples later in this chapter) or in a text format that you can directly manipulate by typing. The text format is also used as part of the export format and for printing.
You can choose either the text or tree representation for expressions at any time using the preferences.