accessing previously created element

I'm going to have a simple sequence of elements:

    a
    a
    :
    :

that will be transformed into a more complex sequence:

    b
        c
        d
    b
        c
        d
    :
    :

unfortunately when transforming the i'th 'a' element to a 'b' (assume i > 1) I also need to access the (i-1)'th 'b' element; that is, the values used to create a 'b' element are dependent upon the current 'a' element being transformed and the preceding 'b' element.  Is this possible?  in a single pass?  

Any hints, suggestions, or pointers to documentation are most appreciated.

Paul




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