[GPS] Oct 16, 2003 - Today it occured to me that I might accidentally set the wrong variable in my Perpheon language translator, and it could cause me to have a nasty debug session, so I decided to come up with a proc that sets a variable but returns an error if it doesn't exist.  

 proc ?set {vPtr val} {
  upvar $vPtr v
  if {![info exists v]} {
   return -code error "error: variable doesn't exist."
  }
  set v $val
 }

Here is a log demonstrating usage:
 % ?set foo bar
 error: variable doesn't exist.
 % set foo abc
 abc
 % ?set foo bar
 bar
 % set foo
 bar
 % 

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