What is the compensation for the complete loss of speech in a person who already had a speech impediment (lisp) unrelated to injury?

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If a person who naturally has a speech impediment loses all their speech due to injury, the compensation due is proportional to the letters lost, provided there is no hope of the natural impediment being removed. However, if the removal of the lisp is expected, such as in the case of a child or an adult capable of correction through teaching, the full *Diyah* is due, based on the presumption that the speech function would have eventually been whole.