%
% stack: name-to-add parse-length
%
  2 copy left
%
% check tree membership at this level
%
% stack: name-to-add parse-length combo
%
  Tree.Member(
  {
%
% the prefix is a member already, keep descending
%
% stack: name-to-add parse-length node
%
   !\
   fromleft
   0
  }
  {
%
% stack: name-to-add parse-length
%
% the prefix does not exist, keep looking
%
  } ifelse