control over nolimits corrections (fonts differ) also with new math constants. Warning: new primitives, format id bumped (HH)

 support of already existing oldmath key in font table plus documenting some yet undocumented properties (no intended functional differences) (HH)

fix for delcode restore; setmath|delcode also accept numbers instead of table (consistent with getters) (HH)

tracker 970: no prev pointer for head and node.check_discretionaries for emergency fixes of messed up disc nodes (better patch)(HH)