anchor.diff:                if all opt.check list then
                   opt.merge list
                 else
  -                throw "One of option ${name} values has a bad type.";
  +                throw "A value of the option `${name}' has a bad type.";
             }
           else opt;
   
  
positive.code: 
  rec {
  
    inherit (lib) isType;
    
  
    isOption = isType "option";
    mkOption = attrs: attrs // {
      _type = "option";
      # name (this is the name of the attributem it is automatically generated by the traversal)
  
negative.code:               if opt.check opt.default then
                  opt.default
                else
                  throw "The default value of the option `${name}' has a bad type.";
            }
          else opt;
  
  