The prime numbers are not regularly spaced. For example from `2` to `3` the step is `1`.
From `3` to `5` the step is `2`. From `7` to `11` it is `4`.
Between 2 and 50 we have the following pairs of 2-steps primes:
`3, 5 - 5, 7, - 11, 13, - 17, 19, - 29, 31, - 41, 43`
We will write a function `step` with parameters:
- `g` (integer >= 2) which indicates the step we are looking for,
- `m` (integer >= 2) which gives the start of the search (m inclusive),
- `n` (integer >= m) which gives the end of the search (n inclusive)
In the example above `step(2, 2, 50)` will return `[3, 5]` which is the first pair between 2 and 50 with a 2-steps.
So this function should return the **first** pair of the two prime numbers spaced with a step of `g`
between the limits `m`, `n` if these g-steps prime numbers exist otherwise `nil` or `null` or `None` or `Nothing` or `[]` or `"0, 0"` or `{0, 0}` or `0 0`(depending on the language). 
#Examples:
- 
`step(2, 5, 7) --> [5, 7] or (5, 7) or {5, 7} or "5 7"`
`step(2, 5, 5) --> nil or ... or [] in Ocaml or {0, 0} in C++`
`step(4, 130, 200) --> [163, 167] or (163, 167) or {163, 167}`
- **See more examples for your language in "RUN"**
- Remarks:
([193, 197] is also such a 4-steps primes between 130 and 200 but it's not the first pair).
`step(6, 100, 110) --> [101, 107]` though there is a prime between 101 and 107 which is 103; the pair 101-103 is a 2-step.
#Notes: 
The idea of "step" is close to that of "gap" but it is not exactly the same. For those interested they can have a look
at . 
A "gap" is more restrictive: there must be no primes in between 
(101-107 is a "step" but not a "gap". Next kata will be about "gaps":-).
For Go: nil slice is expected when there are no `step` between m and n.
Example: step(2,4900,4919) --> nil
import math
def step(g, m, n):

    def prime(n):
        if n >= 2:
            for i in range(2, int(math.sqrt(n)) + 1):
                if not (n % i):
                    return False
        else:
            return False
        return True

    for i in range(m,n-g):
        if prime(i + g) and prime(i):
            return [i, i + g]
import math
def step(g, m, n):

    def prime(n):
        if n >= 2:
            for i in range(2, int(math.sqrt(n)) + 1):
                if not (n % i):
                    return False
        else:
            return False
        return True

    for i in range(m,n-g):
        if prime(i + g) and prime(i):
            return [i, i + g]