If a repaired diagram is re-checked, errors may still be detected, but these are the irreparable complex errors which the user should attempt to fix prior to submitting the diagram to the automatic marker. Other errors will be repaired on the basis of the most likely error – there are some patterns which occur frequently and have highly likely causes (see Figure 4). Simple errors, by definition, are those that can be repaired without changing the essential meaning of the diagram. Not all errors are repaired – only those that can be dealt with with reasonable certainty.

Having identified errors, the tool can be asked to repair the errors (see Figure 3). A textual description of an error can be obtained by right-clicking on the erroneous element and requesting feedback as shown in Figure 2. The syntax checker identifies errors by shading (objects and activations) and change of colour (messages) – although this mechanism may change after usability testing. This facility can be disabled if required, for example if the tool is used in an exam.

tool shown in Figure 1 incorporates a syntax checking facility (within the Tools menu).

Drawing sequence diagram online