Tests and docs for function operator

editions/test/tiddlers/tests/data/functions/FunctionOperator.tid

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+title: Functions/FunctionOperator
+description: Calling a function via the function operator
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Output
+
+\whitespace trim
+\function .dividebysomething(factor:0.5)
+[divide<factor>]
+\end
+
+\function multiplebysomething(first:ignored,factor:2)
+[multiply<factor>multiply[2].dividebysomething[0.25]]
+\end
+
+<$text text={{{ [[4]function[multiplebysomething]] }}}/>
+|
+<$text text={{{ [[6]function[multiplebysomething],[ignored],[4]] }}}/>
+
++
+title: ExpectedResult
+
+<p>64|192</p>

editions/test/tiddlers/tests/data/functions/MissingFunction.tid

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+title: Functions/MissingFunction
+description: Calling a missing function via the function operator
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Output
+
+\whitespace trim
+
+<$text text={{{ [[23]function[missing]] }}}/>
+
++
+title: ExpectedResult
+
+23

editions/tw5.com/tiddlers/filters/function.tid

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+caption: function
+created: 20220909111836951
+modified: 20220909111836951
+op-input: a [[selection of titles|Title Selection]] passed as input to the function <<.place F>>
+op-output: the [[selection of titles|Title Selection]] returned from the function <<.place F>>
+op-parameter: first parameter is the [[function name|Functions]], subsequent parameters are passed to the function by position
+op-parameter-name: F
+op-purpose: apply a [[function|Functions]] to the input list, and return the result
+tags: [[Filter Operators]]
+title: function Operator
+type: text/vnd.tiddlywiki
+
+<<.from-version "5.3.0">> The <<.op function>> operator applies a named [[function|Functions]] to the input titles, and returns the results from the function.
+
+The first parameter of the <<.op function>> operator specifies the name of the function to be called. Subsequent parameters are passed to the function.
+
+The mapping between the parameters is //positional//, with each consecutive parameter specified in the function call mapped to the corresponding parameter in the function definition. Any parameters that are not provided are given their default values.
+
+<<.tip "Compare with the similar [[filter|filter Operator]] and [[subfilter|subfilter Operator]] operators which take a filter strings as their parameter instead of a named function, and does not permit parameters to be passed">>
+
+<<.operator-examples "filter">>