The pseudo-method is_a? determines whether a 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.