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!)