observe(string) {
        let tokens = this._split(string);
        for (let i = 0; i < tokens.length; i++) {
            this._priorValues[tokens[i]] = this._options.prior;
        }
        tokens = this._prefix.concat(tokens).concat(this._suffix); /* add boundary symbols */
        for (let i = this._options.order; i < tokens.length; i++) {
            let context = tokens.slice(i - this._options.order, i);
            let event = tokens[i];
            for (let j = 0; j < context.length; j++) {
                let subcontext = context.slice(j);
                this._observeEvent(subcontext, event);
            }
        }
    }
Observe (learn) a string from a training set