Правила приорите символов в Scala

Precedence Rules
The precedence of an operator is determined by its first character.
The following table lists the characters in increasing order of priority
precedence:
(all letters)
|
^
&
< >
= !
:
+ -
* / %
(all other special characters)

Поэтому a + b ^? c ?^ d less a ==> b | c

 ((a + b) ^? (c ?^ d)) less ((a ==> b) | c)