If a Blackbox (BB) function modifies variable that is also used inside
of a non-blackbox function the following problem occurs: the variable
cannot be neither in the blackbox package neither in the normal
package.

We should try to resolve this problem by :
   (1) analysis wether variables are exclusively used by BB-fun
   (2) pass setter as additionnal argument to BB-functions

Still for now it is preferable to forbbids BB and globals.

* main.go, transform/checktype.go: Here.