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3.1.2 Include user functions User defined functions (functions define by you) can include other user-defined functions into their own definition. If you do that, you should take care of the following things: Changes to the included function cause changes to the including function. Immediately after a change to any function that is included by another, its new definition is valid. You absolutely have to avoid (even indirect) circle-definitions! Never do e.g. this: a(x)=c(x); b(x)=a(x); c(x)=b(x) It is allowed to include a function that does not exist at the time of the inclusion. Graph2D will not give you a warning or something like that. The test will then be done when you calculate values of that function - not before. And if you calculate a value of a function that is defined with the help of a non-existing function you will get undefined values. So take care of that! Examples: "Sum(a,b)=a+b" "f(x)=2^Sum(x,5)+x^2" correct "k=5.6" "foobar(x,y)=k*x+y+1" correct "f(x)=g(x)/6" "g(x)=2^f(x)" error! (circle-definition!) "u(x)=u(1/x)+3.14" error! (circle-definition!)