The may/can/might business

3606c3606
< errmsg("aggregate function calls cannot be nested")));
---
> errmsg("aggregate function calls may not be nested")));

I don't think that this is an improvement, or even correct English.

You have changed a message that states that an action is logically
impossible into one that implies we are arbitrarily refusing to let
the user do something that *could* be done, if only we'd let him.

There is relevant material in the message style guidelines, section
45.3.8: it says that "cannot open file "%s" ... indicates that the
functionality of opening the named file does not exist at all in the
program, or that it's conceptually impossible."

regards, tom lane