def compile(tokens):
    d = {'+':'PLUS', '.':'PRINT', 'I':'I', 'CR':'CR'}
    code_list = []
    branch_stack = []
    words = {}
    start = 0
    i = 0
    while i < len(tokens):
        token = tokens[i]
        if is_int(token):
            codes = ('PUSH', int(token))

        elif token == 'IF':
            branch_stack.append(len(code_list)+1)
            codes = ('IF', None)
        elif token == 'ELSE':
            code_list[branch_stack.pop()] = len(code_list)+2
            branch_stack.append(len(code_list)+1)
            codes = ('ELSE', None)
        elif token == 'THEN':
            code_list[branch_stack.pop()] = len(code_list) + 1
            codes = ('THEN',)

        elif token == 'DO':
            branch_stack.append(len(code_list) + 1)
            codes = ('DO',)
        elif token == 'LOOP':
            codes = ('LOOP', branch_stack.pop())

        elif token == ':':
            words[tokens[i+1]] = len(code_list)
            codes = ()
            i += 1
        elif token == ';':
            start = len(code_list)+1
            codes = ('RET', )

        elif token in d:
            codes = (d[token],)
        elif token in words:
            codes = ('CALL', words[token])

        else:
            pass

        code_list.extend(codes)
        i += 1

    code_list.append('END')
    return code_list, start

def is_int(s):
    try:
        int(s)
    except ValueError:
        return False
    return True