Defines a rule for validating iCalendar content. A rule consists of a validation type (e.g. presence/absence of specific content), and one or more identifiers (names) of iCalendar content. For example, a rule might define a test for one or less DTEND properties using the "OneOrLess" validation type and "DTEND" identifier. Alternatively, a Predicate may be specified to provide additional validation beyond the presence/absence of content. For example, a predicate may be used to ensure certain date properties are DATE/DATE-TIME values. Note that the predicate must specify the negative case, such that if it evaluates true the rule fails.