This patch was created using

  find -name *.v | xargs -L 1 awk -i inplace '{from = 0} /^From/{ from = 1; ever_from = 1} { if (from == 0 && seen == 0 && ever_from == 1) { print "Set Default Proof Using \"Type*\"."; seen = 1 } }1 '

and some minor manual editing

I added the old one in 176a588c but it was never used.

Using this new definition we can express being contractive using a
Proper. This has the following advantages:

- It makes it easier to state that a function with multiple arguments
  is contractive (in all or some arguments).
- A solve_contractive tactic can be implemented by extending the
  solve_proper tactic.

This removes Ralf's hack of using later_car, which is not function in the logic.

Thanks to Aleš for suggesting this.

Use COFEs only for the recursive domain equation solver

It is not non-expansive, so not a function we should use.

This is inspired by ssr, and makes unification faster if it goes
right-to-left.

See https://sympa.inria.fr/sympa/arc/ssreflect/2013-11/msg00010.html

This way, we can use eapply instead of class_apply, which is used
when the instances are defined using the Instance command. It seems
that eapply is stronger as class_apply, and as such solves some issues
when canonical structures have type class parameters, for example:

  Goal Op (option (dec_agree nat)). apply _.

This failed, but is fixed by this commit.

Based on an idea and WIP commits of J-H. Jourdan: the core of a CMRA
A is now a partial function A → option A.

TODO: define sum CMRA
TODO: remove one shot CMRA and define it in terms of sum