Mathematics, Math, Programming, Discrete Mathematics.
{1, 2, 3, 4, 5} (which is FALSE!). This would be true if {1, 2, 3, 4, 5} was instead {1, 2, 3, 4, 5, {1, 3, 4}}. TLDR be careful when checking containment. I like to think of checking containment by removing the outer brackets of the collection we are checking to see is contained and then looking