XML uses three symbols: <, >, and /. And it's a well-known, documented standard. Looking at the last project I depended on, its build.sbt uses :=, %, %%, /, <+=, <<= (in addition to standard Scala symbols like +=, ++=, ++, =>, <=, >=, which an inexperienced developer wouldn't necessarily realise weren't part of SBT's own syntax). That's not an exhaustive list (I've used :+= and :++= as examples in another thread) and I have no idea where to find one. Heck, I've been a professional Scala developer for 5 years and I have literally no idea what <+= or <<= mean.
And there is an oft-documented overuse of custom operators among Scala programmers, however I don't have sufficient expertise to gauge the resulting problems in practice.
