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The rules part contains the rules up to the end of the specification. Each rule is a pattern optionally followed by an action. The pattern is a regular expression. The action, if there is one, is formed of one or more Scheme expressions.
The actions can span over several lines. To distinguish between the rest of the current action and the start of a new rule, SILex checks the indentation. A new rule must start at the beginning of the line. That is, the action starts right after the pattern and contains all the following lines that start with white space.
SILex does not parse the actions. It simply captures the text up to the start of the next rule. So a syntax error in an action is not detected by SILex.
Nevertheless, SILex is able to detect that an action has been omitted. In that case, a default action is supplied.