static ItIt<Int> countIterator_inclusive_step(int a, int b, int step) {
  assertTrue("step > 0", step > 0);
  
  ret new ItIt<Int>() {
    int i = a;
    
    public bool hasNext() { ret i <= b; }
    public Int next() { var j = i; i += step; ret j; }
  };
}

static ItIt<Long> countIterator_inclusive_step(long a, long b, long step) {
  assertTrue("step > 0", step > 0);
  
  ret new ItIt<Long>() {
    long i = a;
    
    public bool hasNext() { ret i <= b; }
    public Long next() { var j = i; i += step; ret j; }
  };
}

static ItIt<Double> countIterator_inclusive_step(double a, double b, double step) {
  assertTrue("step > 0", step > 0);
  
  ret new ItIt<Double>() {
    double i = a;
    
    public bool hasNext() { ret i <= b; }
    public Double next() { var j = i; i += step; ret j; }
  };
}

static <A> ItIt<A> countIterator_inclusive_step(double a, double b, double step, IF1<Double, A> f) {
  ret mapI_if1(f, countIterator_inclusive_step(a, b, step));
}

static <A> ItIt<A> countIterator_inclusive_step(int a, int b, int step, IF1<Int, A> f) {
  ret mapI_if1(f, countIterator_inclusive_step(a, b, step));
}

Began life as a copy of #1018110