#2309: containers: specialize functions that fail in a Monad to Maybe

[Isaac Dupree]

Ross Paterson wrote:
> That's not consensus, but it is a substantial majority, and I think
> we've explored all the issues.  So I propose to make the change.

Well, what are the guidelines in this case?  Proposer's discretion?  If 
a week has gone by and no one has commented further, I guess you should 
commit the change (or ask someone who can, to do so).  At least, the 
ticket shouldn't be closed until either (1) the patches have been 
applied (and this stated), or (2) until it's explicitly (or clearly) 
been dropped with no change made.  If the patches are different from 
what you actually posted before, you might want to first post the actual 
patches you're planning to apply.

-Isaac