In the following sections, variable refers to a predefined TCL variable holding a value that evaluates to a floating point number, region refers to the name of a region, field refers to the name of a field, and function refers to the name of a command created by the function command.
Note that the expressions required by a previous release have been changed to variables. Although the use of variables simplifies reconfiguration of operators and allows constants to be time varying, it also introduces the possibility of difficult to track errors. Watch out for reuse of the same variable name for another purpose. The location linked to a variable name is kept for the lifetime of the interpreter.