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* Do not print `join` in ApplictiveStmt, unless ppr-debug
* Print parens around multiple parallel binds
When ApplicativeDo is enabled, the renamer analyses the statements of a
`do` block and in certain cases marks them as needing to be rewritten
using `join`.
For example, if you have:
```
foo = do
a <- e1
b <- e2
doSomething a b
```
it will be desugared into:
```
foo = join (doSomething <$> e1 <*> e2)
```
After renaming but before desugaring the expression is stored
essentially as:
```
foo = do
[will need join] (a <- e1 | b <- e2)
[no return] doSomething a b
```
Before this change, the pretty printer would print a call to `join`,
even though it is not needed at this stage at all. The expression will be
actually rewritten into one using join only at desugaring, at which
point a literal call to join will be inserted.
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