function _curry1(fn) {
    return function f1(a) {
      if (arguments.length === 0 || _isPlaceholder(a)) {
        return f1;
      } else {
        return fn.apply(this, arguments);
      }
    };
  }
Optimized internal one-arity curry function.

@private
@category Function
@param {Function} fn The function to curry.
@return {Function} The curried function.