an example can be found on AMOSPro_Examples:Examples/H-6/Help_61.amos
Creates a user-DEFined FunctioN, used for the quick calculation of values. It must be given a name, followed by a bracketed list of variables separated by commas. The expression can include any AMOS functions, limited to a single line of your program. (See FN).
Def Fn name(variables)=expression