reject
    | newRules finish toRemove changed numOfElBefore numOfElAfter|
    newRules := rules copy.
    finish := Set new.
    changed := true.
    [changed] whileTrue: [
        changed := false.
        numOfElBefore := (newRules size).
        newRules copy do: [:rule |
            toRemove := true.
            (rule value) do: [:v |
                (nonterminals includes: v)
                ifTrue: [
                    (finish includes: v)
                    ifFalse: [
                        toRemove := false.
                    ]
                ]
            ].
            toRemove
            ifTrue: [
                (rule key) == start
                ifTrue: [
                    ^false
                ].
                finish add: (rule key).
                newRules remove: rule.
            ]
        ].
        numOfElAfter := (newRules size).
        numOfElBefore == numOfElAfter
        ifFalse: [changed := true]
    ].
    ^ true