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The pseudo-method is_a? determines whether an expression's runtime type inherits or includes another type. For example:

a = 1
a.is_a?(Int32)          # => true
a.is_a?(String)         # => false
a.is_a?(Number)         # => true
a.is_a?(Int32 | String) # => true

It is a pseudo-method because the compiler knows about it and it can affect type information, as explained in if var.is_a?(...). Also, it accepts a type that must be known at compile-time as its argument.