SIGNATURE ArrayFold [data]

-- reductions of arrays

SORT data

IMPORT Array[data] ONLY array

FUN / \ : (data**data->data)**array[data]->data         
        -- left and right folding of arrays
        -- informally:
        -- + / [a,b,...,y,z] == a + (b + (...(y + z)...))
        -- + \ [a,b,...,y,z] == z + (y + (...(b + a)...))
        -- undefined for empty arrays