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, they will not be overriden later
> default  := "old default"
> function := { "argument was: " + arg }@(arg: default)
> default  := "new default"
> function()                      == "argument was: old default"

--Rule functions--

In earlier versions of MSE some functions were available in a special ''rule form''.
A call to for example @replace_rule(match:"abc",replace:"xyz")@ is equivalent to @replace@@(match:"abc",replace:"xyz")@ .
For backwards compatability these functions are still available, but they should not be used for new templates.

Programming in a ''rule style'' is still very useful.
A rule is like a normal function with all parameters given, except for the @input@.
These rules can then be combined using the @+@ operator, for example:
> # First all "a"s are replaced, then all "b"s.
> remove_as_and_bs := replace@(match: "a", replace: "") +
>                     replace@(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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