thear.erl

-module(thear).

-export([simplify/1]).

-spec simplify (E) -> E when E::term().

simplify (E) ->
    S = fun
        ({add,E1,{num,0}}) ->
            S(E1);
        ({add,{num,0},E2}) ->
            S(E2);
        …
    end,
    case S(E) of
        E -> E;
        R -> simplify(R)
    end.