Default arguments

It is possible to declare default arguments for functions using the @@@@ operator.
> function := { "argument was: " + arg }@(arg:"default")
If this function is called without the @arg@ argument, then the default value @"default"@ is used instead.
For example:
> function()                      == "argument was: default"
> function(arg: "something else") == "argument was: something else"

For determining whether the argument is set only explicit arguments count, not everything in scope, so
> arg := "something else"
> function()                      == "argument was: default"

//Defaults are evaluated at the time the @@@@ operator is evaluated, so they can be used to simulate static scoping.

--Rule functions--

Some functions are available in ''rule form''.

These rule form functions are functions that create a new [[type:function]] with some defaults filled in.
That new function, the rule, applies some transformation to the input and returns the result.

A rule is like a normal function with all parameters given, except for the @input@.

Rules are often combined using the + operator, for example:
> # First all "a"s are replaced, then all "b"s.
> remove_as_and_bs := replace_rule(match: "a", replace: "") +
>                     replace_rule(match: "b", replace: "")
> 
> text_with_as_and_bs := "bla bla bla"
> text_without_as_and_bs := remove_as_and_bs(text_with_as_and_bs)


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