you run the file directly in ruby right? "-" means it was defined in 
ruby interpreter itself and not in a file.

irb would show "(irb)"


String.instance_method(:to_str) # => #<UnboundMethod: String#to_str>
String.instance_method(:to_str).source_location
# => nil

--in the above case - why does the `nil` come ?

like you already said: "nil" means native, so to_str is defined in C for 
the MRI interpreter

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