Defined expression

Defined expression is taken as an identifier, possibly followed by one more mores accessors, and checks if the referenced value exists, i.e., it is not ``undefined´´. Operator returns boolean to indicate that the exact behaviour is specific to the class.

This operation might invoke class callbacks ``_check(index)´´, ``_defined(name)´´ or ``_defined_<name>()´´.

  defined-expr := "defined" identifier  
  defined-expr := "defined" "(" identifier ")" 

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