Should multi-head arrows be allowed to have different heads?

[jakebeal]

When multi-head and multi-tail arrows were introduced in SEP V013, it was conservatively decided that the heads of a multi-head arrow should all be the same. In other words, if you want to say that a species stimulates on thing and inhibits another thing, you have to use two different arrows to do it.

Recent discussion on SEP V018, however, raised the question of whether this restriction is actually necessary. As the issue is separate from that of SEP V018, I am opening the question as a new issue for discussion here.

I see the following advantages and disadvantages in the current state:

• Keeping distinct relationships on separate arrows is clearer.
• Interaction nodes already mean that an arrow can have two meanings, one for its head and one for its tail. Thus, allowing multiple heads doesn't change the situation much (this might need to be explicitly clarified in any case)
• If there are multiple relationships that could be consolidated, forcing them to be in separate arrows could make a diagram more crowded than strictly necessary.