Add new rule to the ruleset called “Planetary Focus” which says

Each student may have a planetary focus which is tracked in the GNDT in a column called “Focus”.
A student may occasionally change eir focus to any planet currently listed in Rule 2.2 “Planets” or to “No Focus”.
When a student has selected a Focus, it is said that e is “Focusing on” that planet.
New students start with “No Focus”.

Add a subset of this rule called “The effects of focus” which says

Often, if the planet which a student is focusing on is Dignified, then e may increase eir sanity by one.
Often, if the planet which a student is focusing on is Detriment, then any other student may decrease that student’s sanity by one.
If another student is studying a planet which is in Mutual Reception to the planet which a student is focusing on; then that student may change the focus of the other student to match eir own focus.