The most difficult problem that the user faces is that the formula parser sometimes fails to recognise the user's formula. If this happens, the user has to rearrange their formula so that it is parsable.
If the system is unable to parse the formula, the system shows the ``best'' parse found so far by boxing sections for their formula. Figure 4.10 shows the display that the system gives the user when it is unable to parse the user's formula. In this example, there is a single limit on the integral, but the parser's grammar does not accept this construct. As a result, the best that the parser was able to do was reduce the underlying graph to two nodes, these nodes being one for the integral itself, and one for the extra limit. This is displayed to the user by outlining the bounding boxes of these two nodes, these outlines overlaid on the user's input.
The graph grammar based parser allows for some leniency in the placement of characters so it usually parses formulae that are part of the grammar on its first attempt. Nonetheless, significant deviations in placement from what the grammar expects can cause parsing failures. In such a case, the user must manually realign the input characters by using the select and move mode.
Other problems that lead to the misparsing of a formula include misgrouped or misrecognised characters that are not noticed by the user. When the formula does not parse, the user has to find these mistakes and correct them using the modify stroke groups or modify character modes.
![\includegraphics[width=0.95\linewidth]{figures/cant_parse.eps}](img100.gif)