more general notion of command span: command keyword not necessarily at start;
support for special 'private' prefix for local_theory commands;
clarified parse_spans, with related operations for document Thy_Output and editor Token_Markup;
src/Pure
src/FOL
src/HOL
src/ZF
src/CCL
src/CTT
src/Cube
src/FOLP
src/LCF
src/Sequents
src/Doc
src/Tools