Just like or is almost only be useful in structures of the form

func arg
  or stmt;
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err would almost only be useful in structures of the form

func arg
  err stmt;
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Simply replacing err with // would be wrong, but appropriate use of parens will do the trick:

func(arg)
   // stmt;
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(func arg)
   // stmt;
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if (not defined func arg) {
   stmt;
}
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etc.