support fragment of justification diagram

[mosser]

We need to modify the grammar so that one can express "fragments" of justification diagrams, and weave them into existing justification to support off-the-shelf reuse of justification sequences.

[mosser]

After discussion with @MireilleBF and examples from @BryanSun-24, we ended-up changing the approach and moving for a merge instead of a weave. No need for fragments so far, it looks like merging elements should be sufficient.