In my work I use a common error page to display an error message, But it is a page of the activity so it is not addressed by multiple activities. I do not see why you couldn't have a common "Error Activity" with a single or multiple error pages. The only restriction would be that parameters that were passed to this activity should be stored as System Globals using System.setGlobal and System.getGlobal to hold the things the page needs to display.

If a variable or object holds a number either as a real number or as a string which is clearly a number (e.g. "5") then the incrementation and decrementation operators should always work as ++ and -- are not overloaded as the plus sign is. I have not tried it for the numeric string case but the incrementation and decrementation operators should force an auto conversion just as the case for a="5" B=5*a, (or b=a*5) System.print("B="+b) should print B=25