The Kraaij-Pohlmann stemming algorithm

strings ( ch )
integers ( p1 p2 )
booleans ( Y_found stemmed GE_removed )

routines (

   R1 R2
   C V VX
   lengthen_V
   Step_1 Step_2 Step_3 Step_4 Step_7
   Step_6 Step_1c
   Lose_prefix
   Lose_infix
   measure
)

externals ( stem )

groupings ( v v_WX AOU AIOU )

stringescapes {}

define v        'aeiouy'
define v_WX     v + 'wx'
define AOU      'aou'
define AIOU     'aiou'

backwardmode (

    define R1 as ($p1 <= cursor)
    define R2 as ($p2 <= cursor)

    define V  as test (v or 'ij')
    define VX as test (next v or 'ij')
    define C  as test (not 'ij' non-v)

    define lengthen_V as do (
        non-v_WX [ (AOU] test (non-v or atlimit)) or
                   ('e'] test (non-v or atlimit
                               not AIOU
                               not (next AIOU non-v)))
        ->ch insert ch
    )

    define Step_1 as
    (
        [substring] among (

            '{'}s' (delete)
            's'    (R1 not ('t' R1) C delete)
            'ies'  (R1 <-'ie')
            'es'
                   (('ar' R1 C ] delete lengthen_V) or
                    ('er' R1 C ] delete) or
                    (R1 C <-'e'))

            'aus'  (R1 V <-'au')
            'en'   (('hed' R1 ] <-'heid') or
                    ('nd' delete) or
                    ('d' R1 C ] delete) or
                    ('i' or 'j' V delete) or
                    (R1 C delete lengthen_V))
            'nde'  (<-'nd')
        )
    )

    define Step_2 as
    (
        [substring] among (
            'je'   (('{'}t' ] delete) or
                    ('et'   ] R1 C delete) or
                    ('rnt'  ] <-'rn') or
                    ('t'    ] R1 VX delete) or
                    ('ink'  ] <-'ing') or
                    ('mp'   ] <-'m') or
                    ('{'}'  ] R1 delete) or
                    (] R1 C delete))
            'ge'   (R1 <-'g')
            'lijke'(R1 <-'lijk')
            'ische'(R1 <-'isch')
            'de'   (R1 C delete)
            'te'   (R1 <-'t')
            'se'   (R1 <-'s')
            're'   (R1 <-'r')
            'le'   (R1 delete attach 'l' lengthen_V)
            'ene'  (R1 C delete attach 'en' lengthen_V)
            'ieve' (R1 C <-'ief')
        )
    )

    define Step_3 as
    (
        [substring] among (
            'atie'  (R1 <-'eer')
            'iteit' (R1 delete lengthen_V)
            'heid'
            'sel'
            'ster'  (R1 delete)
            'rder'  (<-'r')
            'ing'
            'isme'
            'erij'  (R1 delete lengthen_V)
            'arij'  (R1 C <-'aar')
            'fie'   (R2 delete attach 'f' lengthen_V)
            'gie'   (R2 delete attach 'g' lengthen_V)
            'tst'   (R1 C <-'t')
            'dst'   (R1 C <-'d')
        )
    )

    define Step_4 as
    (
        (   [substring] among (
                'ioneel'  (R1 <-'ie')
                'atief'   (R1 <-'eer')
                'baar'    (R1 delete)
                'naar'    (R1 V <-'n')
                'laar'    (R1 V <-'l')
                'raar'    (R1 V <-'r')
                'tant'    (R1 <-'teer')
                'lijker'
                'lijkst'  (R1 <-'lijk')
                'achtig'
                'achtiger'
                'achtigst'(R1 delete)
                'eriger'
                'erigst'
                'erig'
                'end'     (R1 C delete lengthen_V)
            )
        )
        or
        (   [substring] among (
                'iger'
                'igst'
                'ig'      (R1 C delete lengthen_V)
            )
        )
    )

    define Step_7 as
    (
        [substring] among (
            'kt'   (<-'k')
            'ft'   (<-'f')
            'pt'   (<-'p')
        )
    )

    define Step_6 as
    (
        [substring] among (
            'bb'   (<-'b')
            'cc'   (<-'c')
            'dd'   (<-'d')
            'ff'   (<-'f')
            'gg'   (<-'g')
            'hh'   (<-'h')
            'jj'   (<-'j')
            'kk'   (<-'k')
            'll'   (<-'l')
            'mm'   (<-'m')
            'nn'   (<-'n')
            'pp'   (<-'p')
            'qq'   (<-'q')
            'rr'   (<-'r')
            'ss'   (<-'s')
            'tt'   (<-'t')
            'vv'   (<-'v')
            'ww'   (<-'w')
            'xx'   (<-'x')
            'zz'   (<-'z')
            'v'    (<-'f')
            'z'    (<-'s')
        )
    )

    define Step_1c as
    (
        [substring] among ( (R1 C)
            'd' (not ('n' R1) delete)
            't' (not ('h' R1) delete)
        )
    )
)

define Lose_prefix as (
    ['ge'] test hop 3 (goto v goto non-v)
    set GE_removed
    delete
)

define Lose_infix as (
    next
    gopast (['ge']) test hop 3 (goto v goto non-v)
    set GE_removed
    delete
)

define measure as (
    $p1 = limit
    $p2 = limit
    do(
        repeat non-v  atleast 1 ('ij' or v)  non-v  setmark p1
        repeat non-v  atleast 1 ('ij' or v)  non-v  setmark p2
    )

)
define stem as (

    unset Y_found
    unset stemmed
    do ( ['y'] <-'Y' set Y_found )
    do repeat(goto (v  ['y'])<-'Y' set Y_found )

    measure

    backwards (
            do (Step_1 set stemmed )
            do (Step_2 set stemmed )
            do (Step_3 set stemmed )
            do (Step_4 set stemmed )
    )
    unset GE_removed
    do (Lose_prefix and measure)
    backwards (
            do (GE_removed Step_1c)
        )
    unset GE_removed
    do (Lose_infix and measure)
    backwards (
            do (GE_removed Step_1c)
        )
    backwards (
            do (Step_7 set stemmed )
            do (stemmed or GE_removed Step_6)
        )
    do(Y_found  repeat(goto (['Y']) <-'y'))
)