static void skip_whitespace(TextReader reader) {
    while(Char.IsWhiteSpace((char) reader.Peek())) reader.Read();
}

static AST parse_application(TextReader reader) {
    skip_whitespace(reader);
    var asts = new List<AST>();

    while(!(reader.Peek() == -1 || ((char) reader.Peek()) == ')')) {
        asts.Add(parse_term(reader));
        skip_whitespace(reader);
    }

    if(asts.Count == 0) return null;

    if(asts.Count == 1) return asts[0];

    return A(asts.ToArray());
}

static AST parse_term(TextReader reader) {
    var c = (char) reader.Peek();

    switch(c) {
        case '/':
            return parse_abstraction(reader);
        case '(':
            reader.Read();
            var res = parse_application(reader);
            reader.Read(); // Read ')'
            return res;
        default:
            return parse_variable(reader);
    }

    throw new Exception("Tried to parse invalid term.");
}

static AST parse_abstraction(TextReader reader) {
    reader.Read();

    skip_whitespace(reader);
    var variable = parse_variable(reader);

    skip_whitespace(reader);
    reader.Read(); // TODO: Check for '.'
    skip_whitespace(reader);

    var body = parse_application(reader);

    return new AbstractionAST(variable, body);
}

static VariableAST parse_variable(TextReader reader) {
    var builder = new StringWriter();

    while(Char.IsLetterOrDigit((char) reader.Peek())) {
        builder.Write((char) reader.Read());
    }

    return new VariableAST(builder.ToString());
}