It is a good metaphor indeed, but it appears to be a spider that bites....: the next step in the construction showed me that the tops of the triangles should have been not less than 9 mm closer to one another and for this reason the connecting bar 28+AN is far to short. I will try to fix the problem and correct the angles of the triangles without damaging them.
But how far am I supposed to 'think ahead' in a step by step instruction?