Interaction designers typically revise user interface prototypes by adding unstructured notes to storyboards and screen printouts. How might computational tools increase the efficacy of UI revision? This paper introduces d.note, a revision tool for user interfaces expressed as control flow diagrams. d.note introduces a command set for modifying and annotating both appearance and behavior of user interfaces; it also defines execution semantics so proposed changes can be tested immediately. The paper reports two studies that compare production and interpretation of revisions in d.note to freeform sketching on static images (the status quo). The revision production study showed that testing of ideas during the revision process led to more concrete revisions, but that the tool also affected the type and number of suggested changes. The revision interpretation study showed that d.note revisions required fewer clarifications, and that additional techniques for expressing revision intent could be beneficial.