text = "hello+there$" # explicit end-of-text marker
position = 0
lval = None

def parseLetter(): # a | b | ... | y | z
    global text, position, lval
    if text[position] < "a" or text[position] > "z": return False
    lval = text[position]
    position += 1
    return True

def parseIdentifier(): # letter | letter identifier
    global text, position, lval
    start = position
    if not parseLetter(): return False
    parseIdentifier()
    lval = ["identifier", text[start:position]]
    return True

def parseSum(): # sum + identifier | identifier = identifier ( + identifier )*
    global text, position, lval
    if not parseIdentifier(): return False
    saved = position
    result = lval
    while text[position] == "+":
        position += 1
        if not parseIdentifier():
            print("identifier expected at position " + str(position))
            exit()
        result = ["add", result, lval]
        saved = position
    lval = result
    return True

print(parseSum())
print(position)
print(lval)