1+ is not R5RS

[Bertalan Fodor (LilyPondTool)]

Hi,

I'm trying to implement a more or less Guile compatible Scheme 
implementation in Java.
However I found that Guile allows 1+ 1- etc. as identifiers. However, 
the R5RS spec does not allow.

Could you provide me some background why 1+ exists, as it is the same as 
(+ 1, and why is it named like this?

Thanks,

Bert