third 2 1 mid ":" eq$
 {
%
% this is an increment of var no stack
%
  third 2 left &"x"
 }
 {
  third length) 1 eq
  {
%
% operates on stack
%
   third
  }
  {
%
% has embedded name
%
   third 1 left &"x"
  } ifelse
 } ifelse