anchor.diff:  
         throwEvalHelp = { reason , errormsg ? "" }:
           throw (''
  -          Package ‘${attrs.name or "«name-missing»"}’ in ${pos''} ${errormsg}, refusing to evaluate.
  +          Package ‘${attrs.name or "«name-missing»"}’ in ${pos_str} ${errormsg}, refusing to evaluate.
   
             '' + ((builtins.getAttr reason remediation) attrs));
   
  
positive.code:         }
          // attrs.meta or {}
            # Fill `meta.position` to identify the source location of the package.
          // lib.optionalAttrs (pos' != null)
            { position = pos'.file + ":" + toString pos'.line; }
          ;
  
      in
  
negative.code: 
        # Throw an error if trying to evaluate an non-valid derivation
        validityCondition =
               let v = checkValidity attrs;
               in if !v.valid
                 then throwEvalHelp (removeAttrs v ["valid"])
                 else true;
  