class Interval { from to by }
    method do: block
       let i = from.
       { i <= to }
           whileTrue: { block value: i. i = i + by }.
       self
    method size
       by == 1
          ifTrue: { 1 + to - from }
          ifFalse: { (1 + to - from) asInteger / by }
    method displayOn: stream
       by == 1
         ifTrue: { stream print: "#<Interval {from} to: {to}>" }
         ifFalse: { stream print: "#<Interval {from} to: {to} by: {by}>" }
    method printOn: stream
       self displayOn: stream
end