-spec mktree(Signal) -> Tree when
        Signal  :: gsc:signal(),
        Tree    :: gsc_ntree:ntree().

% @doc make into a tree
mktree(Sig) ->
    Tree0 = gsc_ntree:nstem(vtokens, Sig),
    Tree1 = rerootl_tkstr("=>", Tree0),
    Tree2 = rerootl_tkstr("*", Tree1),
    Tree2.


rerootl_tkstr(S, Tree0 = #ns{val = Root0}) ->
    Kids0 = gsc_ntree:deleaf0(Tree0),
    IsntS = fun(Tk) -> isnt_str(S, Tk) end,
    case lists:splitwith(IsntS, Kids0) of
        % found
        % input:
        %   *s Root0
        %   |
        %   +-- .l Foo
        %   +-- .l "=>"
        %   +-- .l Bar
        % output:
        %   *s "=>"
        %   |
        %   +-- *s Root0 -- .l Foo
        %   +-- *s Root0 -- .l Bar
        {LHS1, [Tk0 | RHS1]} ->
            Root1    = Root0,
            LTree1   = gsc_ntree:releaf0(Root1, LHS1),
            RTree1   = rerootl_tkstr(S, gsc_ntree:releaf0(Root1, RHS1)),
            NewRoot0 = {op, Tk0},
            NewKids0 = [LTree1, RTree1],
            NewTree  = gsc_ntree:releaf0(NewRoot0, NewKids0),
            NewTree;
        % not found, nothing to do
        {Kids0, []} ->
            Tree0
    end.


%reroot_mapsto(Tree0 = #ns{val = Root0}) ->
%    Kids0 = gsc_ntree:deleaf0(Tree0),
%    IsntMapsto = fun(DL) -> isnt_str("=>", Tk) end,
%    case lists:splitwith(IsntMapsto, Kids0) of
%        % found
%        {LHS1, [Tk0 | RHS1]} ->
%            Root1    = Root0,
%            LTree1   = gsc_ntree:releaf0(Root1, LHS1),
%            RTree1   = reroot_mapsto(gsc_ntree:releaf0(Root1, RHS1)),
%            NewRoot0 = {op, Tk0},
%            NewKids0 = [LTree1, RTree1],
%            NewTree  = gsc_ntree:releaf0(NewRoot0, NewKids0),
%            NewTree;
%        % nothing to do
%        {Kids0, []} ->
%            Tree0
%    end.



isnt_str(X, Y) ->
    not is_str(X, Y).

is_str(S, #tk{str = S}) -> true;
is_str(_, _)            -> false.