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Mixin Class Safety

Previous Stack example does not check for underruns. Implement "Saftety" as separate class, which can be used as a mixin.

#
# Create a safety class
#
Class Safety
Safety instproc init {} {
  my set count 0
  next
}
Safety instproc push {thing} {
  my incr count
  next
}
  
Safety instproc pop {} {
  if {[my set count] == 0} then { error "Stack empty!" }
  my incr count -1
  next
}