Ruby Proc responds to `===`, so `my_proc === arg` is equivalent to `my_proc.call(arg)`. Present since at least Ruby 1.9.1. Because Ruby's `case/when` dispatches via `===`, this lets you use procs directly as `when` conditions (supported since Ruby 2.2). With arguments, `proc === (a, b)` doesn't parse with the operator sugar, but `proc.===(a, b)` works as a normal method call, as does `proc.curry[a][b]` and `proc.call(a, b)`.