This is the core functionality from https://github.com/bluegraybox/examples/blob/first-concurrent/indent-parser/parse_indents.erl

parse_indents.erl

build_nodes(Parent, Group, []) -> Parent ! {Group, []};
build_nodes(Parent, Group, Lines) ->
    [First|Rest] = Lines,
    %% split off our children from the rest of the Lines.
    %% the first line with an indent =< ours is a sibling, not a child
    Criterion = fun(X) -> X#line.indent =< First#line.indent end,
    [ChildLines, SiblingLines] = split_list(Criterion, Rest),
    spawn(?MODULE, build_nodes, [self(), children, ChildLines]),
    spawn(?MODULE, build_nodes, [self(), siblings, SiblingLines]),
    receive
        {children, Children} ->
            Node = #node{content=First#line.content, children=Children}
    end,
    receive
        {siblings, Siblings} ->
            io:fwrite( "[~s] ~p~n", [Node#node.content, self()]),
            Parent ! {Group, [Node|Siblings]}
    end.

Here's the full code for the concurrent Erlang version of the indent parser. There's also a sequential Erlang version and a Lisp version.