Add set math

[danielmorrison]

Things like

Superset ⊃
Subset ⊂
Does not contain ∌
etc.

[Aditya-Chowdhry]

Hi, should I do this?
Superset ⊃
Will it return true or false ?
for example: A= {1,2,3,4} B={1,2,3}
Therefore, A ⊃ B => true
A ⊂ B => false
A ∌ B => false

[danielmorrison]

Seems right to me, go for it!

Thanks for your contributions! Your other two PRs are now released in gem 1.4.0.

[Aditya-Chowdhry]

Thanks! I have added these in the latest PR.