fmod ARRAY is

    sorts Array Index Data .
    
    op _[_] : Array Index -> Data .
    op _[_/_] : Array Data Index -> Array .
    
    var A : Array .
    vars I J : Index .
    var D : Data .
    
    *** For reductions only
    vars I1 I2 I3 : Index .
    vars D1 D2 : Data .
    
    eq A[D / I][I] = D .
    eq A[D / I][J] = A[J] [owise] .
endfm

*** Change element I1 to value D1, and
*** change element I2 to value D2, then
*** look up I1 - it should be D1
red A[D1 / I1][D2 / I2][I1] .

*** Make the same changes and then look
*** up I2 - it should be D2
red A[D1 / I1][D2 / I2][I2] .

*** Same changes, but look up I3 (not
*** changed) - it should be A[I3]
red A[D1 / I1][D2 / I2][I3] .