[IDEA] pragma to assign variables by evaluating a filter expression

Currently the only way to assign a variable using a filter that can be imported elsewhere is to use nested $set widgets and the syntax is a frequent pitfall, especially in terms of understanding the requirements for nesting.

It is recommended to introduce a new pragma shorthand so that user can use set widgets to assign variables using a familiar syntax like procedures and functions.

Possible syntax:

\set myvar() [tag[HelloThere]]

or

\function-now myvar() [tag[HelloThere]] \function-now myvar(daysPerYear:365) [tag<daysPerYear>]

equivalent to: <$set name="myvar" filter="[tag[HelloThere]]"></$set>

The latter has the advantage of emphasizing the similarity with functions, the difference being when the execution of the filter happens.

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Update:

• Assign MVV as opposed to a title list
• consider alternative syntax \let myvar= [tag[HelloThere]] for consistency with variable assignment in filter expressions.