A reaction can occur in two directions:
But it can be the case that the reaction occurs in both directions at the same time:
It can also be the case that the reaction occurs in an unknown or unspecified direction:
This set of four reactions are tightly related, as they share the
same participants. Any change in one of the reactions should be
reflected in the other three, and their textual representations and
fingerprints updated accordingly.
In fact, the participants are stored in the database only once,
not four times (not only for space considerations, but for
consistency and to avoid the hassle and errors due to having four
copies of the same data).
So one of them - UN - is considered arbitrarily as the access point
to the participants. That's why we need a UN_REACTION column in the
INTENZ_REACTIONS
table.