Proof nodes with linear position and backtracking (replaces obsolete proof_history.ML).
Proof nodes with linear position and backtracking (replaces obsolete proof_history.ML).

MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle