Arc has a feature called "ssyntax", short for symbol syntax, that allows us to use certain characters in a symbol as shorthand for a commonly used longer form.
In this case, "karma.u" expands to (karma u), which is a macro that looks up the users karma. Functions applied to single arguments are extremely common in arc, so it is quite useful to have an abbreviation.
Other ssyntax include:
a!b -> (a 'b)
~a -> (no a)
a~b -> (compose a (no b))
a:b -> (compose a b) ;(a:b c) is equivalent to (a (b c))
