use automatic type coerctions in Sqrt example

make SML/NJ happy;

not all Nitpick 'constructors' are injective -- careful

added another definition snipped

actually fail on prolog errors -- such as swipl startup failure due to missing shared libraries -- assuming it normally produces clean return code 0;

modifiers for classical wrappers operate on Proof.context instead of claset;

removed historic comments;

tuned exceptions -- avoid composing error messages in low-level situations;