Subsume funind

[Alizter]

At the WG in Sophia we discussed that currently today coq-equations doesn't fully subsume funind so cannot be considered a full replacement for it. What does funind do that equations caan't? Let's record the details here since I've already forgotten from last week.

cc @mattam82 @ppedrot @ybertot

[ybertot]

https://github.com/ybertot/breadth_first_search/tree/Equations_pb

gives an example of usage of Equations that does not terminate gracefully. After a while, the Coq process simply dies, without an error message or an explanation.

Also, I don't understand why the exist constructor has to be prefixed with an @.