Function on_expression_key 

fn on_expression_key<'i>(
    tokens: &mut TokenSlice<'i, Token>,
    key_token: &'i Token,
    encoding: Option<Encoding>,
    receiver: &mut dyn EventReceiver,
    error: &mut dyn ErrorSink,
)

Start an expression from a key compatible token type

expression =  ws [ comment ]
expression =/ ws keyval ws [ comment ]
expression =/ ws table ws [ comment ]

;; Key-Value pairs

keyval = key keyval-sep val