fixed problematic addition operation in the 'approximation' package (previous version used much too high precision sometimes)

new lemmas

discontinue old Ubuntu 18.04 LTS, e.g. it cannot build documentation "prog-prove";

following recent Phabricator update, after 2021 Week 13 (Late March);

merged

Cosmetic: no !! in the lemma statement

clarified README; avoid odd patching of sources;

more standard header, with utf-8 encoding;

clarified HTML template (see also 04cb7e02ca38): avoid odd patching of sources;

merged

clarified signature;

clarified: follow "isabelle version -t";

further clarification of Isabelle distribution identification -- avoid odd patching of sources;

tuned signature -- more explicit types;

more robust and uniform ISABELLE_TAGS;

clarified ISABELLE_ID: distribution vs. hg archive vs. hg repos;

simplified release status (again), in contrast to a43898f76ae9;

more uniform HTTP resources;

new automatic order prover: stateless, complete, verified

clarified (again): local tip could be actually more recent;

tuned;

tuned;

clarified name;

more systematic java_library: avoid empty entries, declaration order as for other bash functions;

support sequential LaTeX jobs: more robust when TeX installation is self-installing packages etc.;

updated to latest latex due to new mechanism for dealing with bold ccfonts

removal of needless hypothesis in hd_rev and last_rev

more robust;

clarified message;

tuned;