Expression

If an expression is non-existent, its value is "". The Expression type is String.
The formatParamSpec's tag "type" can have values EventType|Variable|VariableArray.

<parameter type="Expression">
	<type>String</type><!--many of these types in the future -->
	<formatString>this is %1% from ecs0:var %2%, this is from ecs0:var1 at index str1 %3%, this is from ecs0:var1 at index ev:index %4% </formatString>
	<formatParmSpec> <!--0..N of these, 0 is fixed format string -->
		<type>EventType</type> <!--many of these types in the future -->
		<name>ev:msg</name>
	</formatParmSpec>
	<formatParmSpec> <!--0..N of these, 0 is fixed format string -->
		<type>Variable</type> <!--many of these types in the future -->
		<name>ecs0:var</name>
	</formatParmSpec>
	<formatParmSpec><!--0..N of these, 0 is fixed format string -->
		<type>VariableArray</type> <!--many of these types in the future -->
		<name>ecs0:var1</name>
		<index type="String">str1</index>
	</formatParmSpec>
	<formatParmSpec><!--0..N of these, 0 is fixed format string -->
		<type>VariableArray</type> <!--many of these types in the future -->
		<name>ecs0:var1</name>
		<index type="EventField">ev:index</index>
	</formatParmSpec>
</parameter>

default

<parameter type="Expression">
	<type>String</type>
	<formatString>enter format string</formatString>
</parameter>