NB. The stack object template which inherits a print operation

data =: ''

stack_z_ =: monad define
('operation value') =. 2 take y
if. operation match 'type'
    do. 'stack'
  elseif. operation match 'emptyp'
    do. 0 = tally data
  elseif. operation match 'push'
    do. for_effect_only data =: (box value) , data
  elseif. operation match 'top'
    do. if. 0 = tally data
          do. 'top:  stack is empty'
          else. open first data
        end.
  elseif. operation match 'pop'
    do. if. 0 = tally data
          do. 'pop:  stack is empty'
          else. for_effect_only data =: rest data
        end.
  elseif. operation match 'size'
    do. tally data
  elseif. operation match 'print'
    do. if. 0 = tally data
          do. 'print: stack is empty'
          else. for_effect_only display 'top of stack'
                print 'print' ; box data
        end.
  elseif. 1
    do. root_object operation ; value
end.
)