>function meanangle (a) ...
$  z=sum(exp(rad(a)*I));
$  if z~=0 then error("Not meaningful");
$  else return deg(arg(z))
$  endfunction
>meanangle([350,10])
 0
>meanangle([90,180,270,360])
 Error : Not meaningful

 Error generated by error() command

 Error in function meanangle in line
 if z~=0 then error("Not meaningful");
>meanangle([10,20,30])
 20