** (Advanced) 
** Handed to 'Rule' classes during the matching process. 
mixin RuleCtx {
    ** Call to process a sub-rule. Returns 'true' if it matched successfully.
    abstract Bool process(Rule rule)
    ** Consumes 1 character from the underlying input stream.
    abstract Int readChar()

    ** Returns the current position in the underlying input stream.
    abstract Int currentPos()
    ** Rolls back the underlying input stream to the given position. 
    abstract Void rollbackToPos(Int pos)

    ** Returns 'true' if end-of-stream is reached. 
    abstract Bool eos()

    ** Logs the given message to debug. It is formatted to be the same as the other Pegger debug messages. 
    abstract Void log(Str msg)

    ** Returns a PEG specific 'ParseErr' to be thrown.
    abstract PegParseErr parseErr(Str errMsg)