cpp14

First of all, great library!

I am having some confusion in understanding the role of values_first2 in set_difference_by_key as mentioned here.

In general terms, the result of set difference D = A - B will not contain values from B therefore values_first2 can never be part of D. So `val

Please note that this function has been deprecated in 2011

Add Runnable links of the code below the example.
Use link name Run Code which links to a Online C++ complier conatining the example code.
Use Rextester as the Online C++ Compiler (If you have a better suggesstion please comment).

Follow below steps to generate the Run Code Link.

1. Go to Rextester
2. Copy or write the example