strange behavior when overriding `function`

Created by: jlongster

I wrote a macro that overrides the function keyword:

let function = macro {
    rule { ($param (,) ...) $body } => {
        function ($param (,) ...) $body
    }

    rule { $id:ident ($param (,) ...) $body } => {
        function $id ($param (,) ...) $body
    }
}

macro foo {
    rule { $x } => {
        $x
    }
}

The function macro works fine, but it doesn't work if I define another macro. Evidently macro expands to something that contains this function definition:

function (stx context) {return syntaxCase (stx , context) {case {_ $x} = > {return syntax {$x}}}}

Why don't the arguments have a comma in between them? It makes it strangely unintuitive to write macros that override function :)