string - 字符串处理函数
- centre/2
- centre/3
- chars/2
- chars/3
- chr/2
- concat/2
- copies/2
- cspan/2
- equal/2
- join/2
- left/2
- left/3
- len/1
- rchr/2
- right/2
- right/3
- rstr/2
- span/2
- str/2
- strip/1
- strip/2
- strip/3
- sub_string/2
- sub_string/3
- sub_word/2
- sub_word/3
- substr/2
- substr/3
- to_float/1
- to_integer/1
- to_lower/1
- to_upper/1
- tokens/2
- words/1
- words/2