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A pico is provided with entity variables by its installed rulesets. Entity variables can be assigned values only inside of rule postludes. An entity variable can be assigned a value (computed by an expression), or it can be cleared (line 87).
When an entity variable is mentioned in an expression (which can happen anywhere an expression can appear in a ruleset), and it does not exist in the pico/ruleset then the value is null.
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In the first version of the syntax (line 810), the domain is given as a bare word between the keywords raise and event, and the type is usually a String constant (but can be computed by an expression computing a String which is a valid identifier.)
In the second (and newer) version of the syntax (line 911), the event is identified by a String expression which consists of a valid identifier for the domain followed by a colon followed by a valid identifier for the type.
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