The pseudo-method nil? determines whether an expression's runtime is Nil. For example:

a = 1
a.nil?          # => false

b = nil
b.nil?          # => true

It is a pseudo-method because the compiler knows about it and it can affect type information, as explained in if var.nil?(...).

It has the same effect as writing is_a?(Nil) but it's shorter and easier to read and write.

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