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freeze 

        # Set.freeze

(from ruby core)
### Implementation from Object
---
    obj.freeze    -> obj

---

Prevents further modifications to *obj*. A FrozenError will be raised if
modification is attempted. There is no way to unfreeze a frozen object.
See also Object#frozen?.

This method returns self.

    a = [ "a", "b", "c" ]
    a.freeze
    a << "z"

*produces:*

    prog.rb:3:in `<<': can't modify frozen Array (FrozenError)
     from prog.rb:3

Objects of the following classes are always frozen: Integer, Float,
Symbol.

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