A constraint can be imposed on any two algorithm elements.
To add a constraint element drag it from the Palette to the Scene. The corresponding dialog appears:
<center> <br> <img src="/wiki/download/attachments/4686214/Adding Constraint Element.png"/> <br> </center>
Once the algorithm elements are selected and the OK button is pressed in the dialog, the constraint element is added to the Scene. From now the constraint is binded to the algorithm elements and it is not possible to modify this assignment.