function generator(sequencer) {
  return sequencer.apply(
    null,
    []
      .slice
      .call(arguments)
      .slice(1)
  );
}

function dummySeq() {
  this.next = function() {
    return "dummy";
  };

  return this;
}

function factorialSeq() {
  this.n = 1;
  this.i = 0;

  this.next = function() {
    const value = this.n;

    this.i++;
    this.n *= this.i;

    return value;
  };

  return this;
}

function fibonacciSeq() {
  this.prev = 0;
  this.current = 1;

  this.next = function() {
    const value = this.current;

    const newValue = this.prev + this.current;
    this.prev = this.current;
    this.current= newValue;

    return value;
  };

  return this;
}

function rangeSeq(start, step) {
  this.value = start;
  this.step = step;

  this.next = function() {
    const oldValue = this.value;
    this.value += this.step;

    return oldValue;
  };

  return this;
}

function primeSeq() {
  const isPrime = function(n) {
    const top = Math.floor(Math.sqrt(n));
    for (let i = 2; i <= top; i++) {
      if (n % i == 0) {
        return false;
      }
    }

    return true;
  };

  const nextPrime = function(n) {
    n++;
    while (!isPrime(n)) {
      n++;
    }

    return n;
  };

  this.p = 1;

  this.next = function() {
    this.p = nextPrime(this.p);

    return this.p;
  };

  return this;
}

function partialSumSeq() {
  this.values = arguments;
  this.length = arguments.length;
  this.runningSum = 0;
  this.i = 0;

  this.next = function() {
    if (this.i >= this.length) {
      throw new RangeError("All input was processed!");
    }

    this.runningSum += this.values[this.i++];

    return this.runningSum;
  };

  return this;
}