Optimized internal two-arity curry function.

@private
@category Function
@param {Function} fn The function to curry.
@return {Function} The curried function.
function _curry2(fn) {
	  return function f2(a, b) {
	    switch (arguments.length) {
	      case 0:
	        return f2;
	      case 1:
	        return _isPlaceholder(a) ? f2 : _curry1(function (_b) {
	          return fn(a, _b);
	        });
	      default:
	        return _isPlaceholder(a) && _isPlaceholder(b) ? f2 : _isPlaceholder(a) ? _curry1(function (_a) {
	          return fn(_a, b);
	        }) : _isPlaceholder(b) ? _curry1(function (_b) {
	          return fn(a, _b);
	        }) : fn(a, b);
	    }
	  };
	}