efficiency of using this method of stepping through primes:
  l=1   :  100%
  l=2   :   50%
  l=6   :   33%
  l=30  :   27%
  l=210 :   20%     2*3*5*7
  l=2310:   15%     2*3*5*7*11
  ....
//
2  3  5  7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67
 +1 +2 +2 +4 +2 +4 +2 +4 +6 +2 +6 +4 +2 +4 +6 +6 +2 +6
 start = primes upto p
 total span of step =  product of primes upto (excluding) p
long start[]={2, 3, 5, 7};
long step[]={ 4 ,2 ,4 ,2 ,4 ,6 ,2 ,6 };

l=1

test all numbers: 2, 3, 4, 5, 6, 7, 8, .....

l=2

(most commonly used)
start your prime list with 2
and test all odd numbers: 3, 5, 7, 9, 11, ....
2  3 :  5  7  . 11 13  . 17 19  . 23  .  . 29 31  .  . 37  . 41 43  . 47  .  . 53  .  . 59 61  .  . 67  . 71
 +1  :+2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2 +2

l=6

start your prime list with 2, 3
and test starting from 5, with alternating steps 2, and 4:
7, 11, 13, 17, 19, 23, 25, 29, ...
2  3  5 :  7 11: 13 17: 19 23:  . 29: 31  .: 37 41: 43 47:  . 53:  . 59: 61  .: 67 71
 +1 +2  :+2 +4 :+2 +4 :+2 +4 :+2 +4 :+2 +4 :+2 +4 :+2 +4 :+2 +4 :+2 +4 :+2 +4 :+2 +4

l=30

start your prime list with 2, 3, 5
and test starting from 7, with alternating steps of: +4 +2 +4 +2 +4 +6 +2 +6
2  3  5  7 : 11 13 17 19 23 29 31 37: 41 43 47  . 53 59 61 67: 71 73  . 79 83 89  . 97: 101 103 107 109 113  .   .  127:131 137 139  
 +1 +2 +2  :+4 +2 +4 +2 +4 +6 +2 +6 :+4 +2 +4 +2 +4 +6 +2 +6 :+4 +2 +4 +2 +4 +6 +2 +6 :+4  +2  +4  +2  +4  +6  +2  +6  :

l=210

2   3   5   7  11:  13  17  19  23  29  31  37  41  43  47  53  59  61  67  71  73  79  83  89  97 101 103 107 109 113 127 131 137 139 149 151 157 163 167 173 179 181 191 193 197 199 211 
  1   2   2  4   :2   4   2   4   6   2   6   4   2   4   6   6   2   6   4   2   6   4   6   8   4   2   4   2   4  14   4   6   2  10   2   6   6   4   6   6   2  10   2   4   2  12  6