Infix + triggers ssyntax for ac-andf - even+odd expands to (andf even odd), for example. However, the expander doesn't handle +'s in non-medial position very well - it just cuts them out:
arc> (ssexpand '++foo)
foo
Since medial + is checked before medial dot in ssyntax expansion, this leads '++.n to expand to '.n and thence to (get n).
Albeit it does change the semantics of some "correct" uses of ssyntax slightly (though not in a way I'd expect anyone to rely on); for example, 'foo+bar.baz expands to '(andf foo (bar baz)) under arc3, but expands to '((andf foo bar) baz) with this patch applied.