VC: Syntax error while defining procedures

Question

Requirement is to calculate value of characteristic Volume from the values of characteristics
width, height & depth. To achieve this, defined following source code
$self.volume = $self.width$self.height$self.depth
But could not able to execute condition, All the characteristics are char format & single value.
checked into following threads, which talks about the similar issue, followed their suggestion, but not
able to fix the error.
Re: variant configuration
Author Sathyanarayana naini mentions about
"character format values you have to use strings........ex: "value"", which I am not clear about. Does it
mean, need to maintain string "  " in characteristic value.

Answer 1

All my characteristics are type "char"
error message:
E28021 Error: remaining part of expression cannot be interpreted
After selecting back, cursor blinks on first asterisk.