Changes

Summary

1. avoid duplicate entries: snippet_command is defined within node;
2. merged
3. proper span position for blobs in batch-build (but: practically irrelevant);
4. proper treatment of singleton Position.Offset within blob (amending cb9d5af781b4);
5. more robust batch-build;
6. tuned;
7. tuned signature;
8. clarified signature;
9. tuned signature;
10. clarified signature;
11. tuned;
12. clarified signature;
13. clarified signature, notably access to blob files;
14. support for PIDE markup for auxiliary files ("blobs"); clarified files of theory Pure;
15. tuned;
16. clarified signature --- more explicit types;