op-parameter: <<.from-version "5.3.4">> defaults to: "text". optional: "value". Content type <<.param T>> of the varible content that should be returned
op-purpose: select all values of variables named in the input titles
<<.from-version "5.1.20">> The usual way to retrieve a variable value within a filter is with the angle brackets notation. For example, `[<currentTiddler>]` will retrieve the value of the variable called `currentTiddler`.
The `getvariable` filter operator provides an alternative way to retrieve a variable. For example, `[[currentTiddler]getvariable[]]` is another way to retrieve the value of the variable `currentTiddler`.
The advantage of `getvariable` is that it makes it possible to work with variables whose name is computed, and not known in advance. For example, `[<myvariable>getvariable[]]` gets the value of the variable whose name is given in the variable `myvariable`.