src/lexer/tokens.js
function() {
    const ch = this._input[this.offset - 1];
    const fn = this.tokenTerminals[ch];
    if (fn) {
      return fn.apply(this, []);
    } else {
      return this.yytext;
    }
  }
function() {
    const ch = this._input[this.offset - 1];
    const fn = this.tokenTerminals[ch];
    if (fn) {
      return fn.apply(this, []);
    } else {
      return this.yytext;
    }
  }
reads a custom token