ROOTS

skip ignored commands, similar to former proper_command_at (see d68ea01d5084) -- relevant to Output, Query_Operation etc.;
tuned signature;

src/Pure
src/FOL
src/HOL
src/ZF
src/CCL
src/CTT
src/Cube
src/FOLP
src/LCF
src/Sequents
src/Doc
src/Tools