Question about the oparand. In RhinoPythonPrimer chapter8.4

If u 've looked this primer , maybe u met some mistakes. I'm a students with no foundation in programming(Also our school do not have digital architecture teaching experience,and maybe u know I'm not a native English speaker from grammer i use).

Talk about my matter ,it's really easy.

Debugger deal with the variables Newlength and Length as an error said that can not recongnize the operant style before and after the operator - .I'm think there is no problem  cuz rs.Distance just returns a figure that can do substraction,but it dont.

Thanks.

    Lyung 

(post form I create myself)