17. A system comprising: a user interface device; and one or more computers configured to interact with the user interface device and to: receive text to compose, the text including a plurality of paragraphs; compose the text using paragraph composition, the paragraph composition determining a layout for each paragraph according to a line break scheme that determines line breaks for the paragraph as a whole; designate one or more paragraphs of the plurality of paragraphs as having limited recomposition; receive a modification to a line of a paragraph, the paragraph having a plurality of lines and where the modified line is subsequent to a first line in the paragraph; determine whether the paragraph of the modified line has limited recomposition; if the paragraph has limited recomposition, recompose a portion of the paragraph using a multi-line paragraph composition process without recomposing the full paragraph; and if the paragraph does not have limited recomposition, recomposing the full paragraph, wherein the designating further comprises receiving a user input limiting recomposition for the paragraph, where the user input flags one or more particular lines in the paragraph preventing backing up the composition beyond the flagged one or more lines.
17. A system comprising: a user interface device; and one or more computers configured to interact with the user interface device and to: receive text to compose, the text including a plurality of paragraphs; compose the text using paragraph composition, the paragraph composition determining a layout for each paragraph according to a line break scheme that determines line breaks for the paragraph as a whole; designate one or more paragraphs of the plurality of paragraphs as having limited recomposition; receive a modification to a line of a paragraph, the paragraph having a plurality of lines and where the modified line is subsequent to a first line in the paragraph; determine whether the paragraph of the modified line has limited recomposition; if the paragraph has limited recomposition, recompose a portion of the paragraph using a multi-line paragraph composition process without recomposing the full paragraph; and if the paragraph does not have limited recomposition, recomposing the full paragraph, wherein the designating further comprises receiving a user input limiting recomposition for the paragraph, where the user input flags one or more particular lines in the paragraph preventing backing up the composition beyond the flagged one or more lines. 25. The system of claim 17 , where recomposing includes composing the modified line and each subsequent system in the paragraph.