An apparatus includes a memory and a processor. The memory stores a data structure that includes an array of data blocks, each of which stores one or more strings of text of a document. The processor displays a sequence of elements, each associated with one or more consecutive data blocks of the array, and a sequence of parts, each corresponding to a data block and displaying the strings of text stored by the data block, on a GUI. The processor receives information identifying a modification of a first element, which is associated with a first data block, to which a first part corresponds. The modification is associated with an equivalent modification of the first part. The processor updates the sequence of elements according to the modification and the sequence of parts according to the equivalent modification. The processor converts the updated sequence of elements into an updated data structure.