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Short helpfile for action CUSTOM
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The following arguments are compulsory: 

               PERIODIC - if the output of your function is periodic then you should 
                          specify the periodicity of the function. If the output is not 
                          periodic you must state this using PERIODIC=NO 
                   FUNC - the function you wish to evaluate 
                    ARG - the input to this function. You can use multiple instances 
                          of this keyword i.e. ARG1, ARG2, ARG3... 
                    VAR - the names to give each of the arguments in the function. If 
                          you have up to three arguments in your function you can use x, 
                          y and z to refer to them. Otherwise you must use this flag to 
                          give your variables names. 


