avoid accidental capture of theory value, and thus reduce heap size again (amending 5109e4b2a292);

tuned;

more robust: proper Proofterm.get_proofs_level with bound check;

tuned;

clarified signature: fewer tuples;

clarified signature: fewer tuples;

clarified signature: more explicit get_proofs_level with bounds check;

merged

misc tuning and clarification: more standard Same.commit discipline;

tuned;

tuned names;

more operations; more accurate treatment of beta contraction in norm_term/norm_proof, following Envir.norm_term;

minor performance tuning: more careful treatment of empty environment;

clarified signature;

more zproofs;

clarified signature: support shared cache;

tuned;

tuned: avoid shadowing;

more zproofs;

tuned;

more operations; clarified cache;

tuned signature;

tuned names;

tuned -- eliminate clones;