prefer general Command_Line.tool wrapper (cf. Scala version);

simplified Session_Tree; tuned messages;

some timeouts, which modify the build order;

queue ordering by descending outdegree and timeout;

tuned import;

clarified Session_Tree (with proper integrity check) vs. Queue (with provision for alternative ordering);

clarified Session_Entry vs. Session_Info with related parsing operations; more error positions;

simplified class Job; tuned message;

let with_timing report overall number of threads;

further robustification of interrupts during build;

refined outer syntax;

prefer calligrapic \<RR> \<II> over \<Re> \<Im> for "screen" display (NB: official unicode defines only one version of these glyphs, unlike TeX);