Hello,

I am fuzzing Uniswap v2 and I believe I have found a potential issue with the testPathIndependenceForSwaps test from part 3/4.

When running echidna with testLimit equals to 100k and 4 workers, the test breaks:

swapExactTokensForTokensPathIndependence(uint256): failed!💥
  Call sequence:
    testProvideLiquidityInvariants(7222,148)
    testRemoveLiquidityInvariants(1412263)
    testSwapTokens(878955802143800039497958245)
    testPathIndependenceForSwaps(0)

I believe this is caused by a wrong assumption that swapping back and forth only costs 3% in fees.

    /*
    Swapping x of testToken1 for y token of testToken2 and back should (roughly) give user x of testToken1.
    The following function checks this condition by assessing that the resulting x is no more than 3% from the original x.
    ...
    */
    ...

        assert((x - xOut) * 100 <= 3 * x); // (x - xOut) / x <= 0.03; no more than 3% loss of funds

Instead, each swap takes 3%, so the back-and-forth maximum loss should be 100% - (100%-3%)*(100%-3%) = 5.91%