A special placeholder value used to specify "gaps" within curried functions, allowing partial application of any combination of arguments, regardless of their positions. If g is a curried ternary function and _ is R.__, the following are equivalent: g(1, 2, 3) g(_, 2, 3)(1) g(_, _, 3)(1)(2) g(_, _, 3)(1, 2) g(_, 2, _)(1, 3) g(_, 2)(1)(3) g(_, 2)(1, 3) g(_, 2)(_, 3)(1)