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const_defined? 

        # Module.const_defined?

(from ruby core)
---
    mod.const_defined?(sym, inherit=true)   -> true or false
    mod.const_defined?(str, inherit=true)   -> true or false

---

Says whether *mod* or its ancestors have a constant with the given name:

    Float.const_defined?(:EPSILON)      #=> true, found in Float itself
    Float.const_defined?("String")      #=> true, found in Object (ancestor)
    BasicObject.const_defined?(:Hash)   #=> false

If *mod* is a `Module`, additionally `Object` and its ancestors are
checked:

    Math.const_defined?(:String)   #=> true, found in Object

In each of the checked classes or modules, if the constant is not
present but there is an autoload for it, `true` is returned directly
without autoloading:

    module Admin
      autoload :User, 'admin/user'
    end
    Admin.const_defined?(:User)   #=> true

If the constant is not found the callback `const_missing` is **not**
called and the method returns `false`.

If `inherit` is false, the lookup only checks the constants in the
receiver:

    IO.const_defined?(:SYNC)          #=> true, found in File::Constants (ancestor)
    IO.const_defined?(:SYNC, false)   #=> false, not found in IO itself

In this case, the same logic for autoloading applies.

If the argument is not a valid constant name a `NameError` is raised
with the message "wrong constant name *name*":

    Hash.const_defined? 'foobar'   #=> NameError: wrong constant name foobar

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