Add Echidna tips & tricks

[montyly]

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• If you fuzz a parameter that is expected to have only some values, use %:

function f(uint index) public{
   index = `index%2`
   if (index == 0) {...}
   elif (index == 1) {...}
}

• To fuzz an arbitrary array length, you can use a push/pop approach (explain why?):

uint my_array[]

function push(uint a) public{
  my_array.push(a)
}

function pop() public{
  my_array.pop()
}

[agroce]

Note that in some cases the push/pop approach will tend to produce smaller arrays than you want. Omitting pop can be a good idea (indeed, I would wager a decent chunk of the reason swarm testing works is omitting pop).

[montyly]

@ggrieco-tob : can you add an example for the push/pop example in https://github.com/crytic/building-secure-contracts/blob/master/program-analysis/echidna/fuzzing_tips.md ?

[montyly]

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