mark - puts a mark on the stack
[ 1 2 add ] -> [3]
mark 1 2 add ] -> [3]
A mark is a token which is lying on the stack and
can be found by the user. The mark is used :
1) by using it as command mark
2) by using it as [ when creating an array
3) by using it as << when creating a dict
docu by Marc Oliver Gewaltig and Sirko Straube
/builddir/build/BUILD/nest-simulator-3.3/nest-simulator-3.3/sli/slicontrol.cc