def compile(tokens):
    d = {'+':'PLUS', '.':'PRINT'}
    code_list = []
    if_stack = []
    for token in tokens:
        if is_int(token):
            codes = ('PUSH', int(token))
        elif token == 'IF':
            if_stack.append(len(code_list)+1)
            codes = ('IF', None)
        elif token == 'ELSE':
            code_list[if_stack.pop()] = len(code_list)+2
            if_stack.append(len(code_list)+1)
            codes = ('ELSE', None)
        elif token == 'THEN':
            code_list[if_stack.pop()] = len(code_list) + 1
            codes = ('THEN',)
        else:
            codes = (d[token], )
        code_list.extend(codes)
    code_list.append('END')
    return code_list

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

if __name__ == '__main__':
    tokens = ['3', 'IF', '4', '5', '+', '.', 'ELSE', '1', '2', '+', '.', 'THEN']
    print(compile(tokens))