The most concise suggestion I can think of would be to have rules like:

    rule s
      's' / ''
    end
    rule ies
       'ies' / 'y'
    end
    rule es
      'es'/''
    end

which would let you write things like this:

    rule container
      'can' s / 'box' es / 'bagg' ies
    end

-- Markus